Don't generate types for map entry messages

I'm sure I've implemented this before, but somehow it's been lost in a maze of twisty little branches, all alike.

Introduce C# well-known types

This involves:
- Specifying a namespace in each proto (including ones we'd previously missed)
- Updating the generation script
- Changing codegen to implement IReflectedMessage.Fields explicitly (a good thing anyway)
- Changing reflection tests to take account of the explicit interface implementation

Non-generated code in this commit; generated code to follow

Change the reflection namespace in C#

Update code already within Google.Protobuf.Reflection which was importing Google.Protobuf.DescriptorProtos

Change the C# namespace in descriptor.proto to Google.Protobuf.Reflection.
This then means changing where the generated code lives, which means updating the project file...

It also involves regenerating the C++ - which has updated the well-known types as well,
for no terribly obvious reason...

Generated code changes from previous commit

- Move types into Google.Protobuf.Reflection
- Change codegen to reflect that in generated types

Generated code changes coming in part 2

Pascal-case namespace automatically in C# codegen

Fixes issue 312.

JSON formatting in C#

- Remove the indexers in FieldAccessorTable
- Add a TODO for field ordering in oneof

remove pbconfig from extract script

Change-Id: I5049da7449cbada781aa1e23485a336b584802cb

Added support for JSON and valid input to conformance tests.

enable building nuget package

This was enabled by the recent open-sourcing of JSON
support and MessageDifferencer.

MessageDifferencer allows the conformance suite to expand
because it allows us to write tests for payloads that parse
successfully.  To verify the testee's output payload, we
need to parse it back into a message and compare the message
instances.  Comparing output bytes vs. a golden message is
*not* valid, because protobufs do not have a canonical
encoding (especially in the presence of maps, which have
no prescribed serialization order).

We only add one small JSON test for now, but with the
framework in place we now have the foundation to dramatically
expand the coverage of the conformance test suite.

Also added the ability for the testee to skip tests that
exercise features that are unimplemented.  This allows
Java (which currently has no JSON support) to skip tests
involving JSON.

Change-Id: I697b4363da432b61ae3b638b4287c4cda1af4deb

- Handle oneof properly
- Omit unknown enum values

- No parsing
- Reflection based, so not hugely efficient
- No line breaks or indentation

Improve C# reflection support

Internal local modifications.

Fix-ups suggested in PR #560

... and some implementation changes to go with them.

- The protos are no longer publicly exposed at all
- Oneof detection now works (as we default to -1, not 0)
- OneofDescriptor exposes the fields in the oneof
- Removed unnecessary code for replacing protos - remnant of extensions
- There's now just the non-generic form of IDescriptor

Note that now we need a proto3 version of addressbook.proto. This may affect other platforms, and could do with an overhaul to follow proto3 conventions anyway (e.g. repeated field names). Will need to think about that carefully before merging into master. Raised issue #565 for this.

- Add a partial method called by all constructors
- Generate internal classes for descriptor.proto (only)
- Forbid proto2 descriptors except for descriptor.proto

- FieldAccessorTable is now non-generic
- We don't have a static field per message type in the umbrella class. (Message descriptors are accessed via the file descriptor.)
- Removed the "descriptor assigner" complication from the descriptor fixup; without extensions, we don't need it
- MapField implements IDictionary (more tests would be good...)
- RepeatedField implements IList (more tests would be good)
- Use expression trees to build accessors. (Will need to test this on various platforms... probably need a fallback strategy just using reflection directly.)
- Added FieldDescriptor.IsMap
- Added tests for reflection with generated messages

Changes to generated code coming in next commit.