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

enable building nuget package

- Handle oneof properly
- Omit unknown enum values

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

Improve C# reflection support

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.

- Added new line at the end of SampleEnum
- Moved GeneratedMessageTest.GetSampleMessage to a new class, SampleMessages, and renamed it to CreateFullTestAllTypes.

Fix C# copyright statements

Large changes to repeated field handling

Everything should now be consistent - the only difference between files is the year (aside from generated files).

Fixes issue #531.

This is mostly just making things internal instead of public, removing and reordering a bunch of code in CodedInputStream/CodedOutputStream, and generally tidying up.

Prod code works, but some tests are broken. Obviously those need fixing, then more tests,
and review benchmarks.

Fix for doubly-nested types - issue #307.

No specific test case - if the generated code compiles, the issue is fixed :)