Expose GetUmbrellaClassName in csharp_names.h

Merge C# proto3 from csharp-experimental to master

Throw on null for string/bytes single fields

Rename ThrowHelper to Preconditions and make it public - we'll want to use it from the generated code soon.

Additionally, change it to return the value passed, and make it generic with a class constraint.
A separate method doesn't have the class constraint, for more unusual scenarios.

Remove a few hangers-on from frozenness.

Remove the C# Freeze API

Expose IsClientStreaming and IsServerStreaming in MethodDescriptor

Move the definition of GOOGLE_FALLTHROUGH_INTENDED to port.h

On x64 MSVC platform, macro PROTOBUF_LITTLE_ENDIAN won't be set

First attempt at using profile 259 for Google.Protobuf.

- Fix nupec paths
- Remove an obsolete part of the JSON build
- Add documentation and tests to reflection extension methods, and improve implementations

Update minimal ios and osx version for deployment.

Remove two unused functions.

This requires .NET 4.5, and there are a few compatibility changes required around reflection.
Creating a PR from this to see how our CI systems handle it. Will want to add more documentation,
validation and probably tests before merging.

This is in aid of issue #590.

Expose original binary data for file descriptor

Reflection part 2 - for discussion

I think Jan was actually suggesting keeping both, but that feels redundant to me. The test diff is misleading here IMO, because I wouldn't expect real code using reflection to use several accessors one after another like this, unless it was within a loop. Evidence to the contrary would be welcome :)

This change also incidentally goes part way to fixing the issue of the JSON formatter not writing out the fields in field number order - with this change, it does except for oneofs, which we can fix in a follow-up change.

I haven't actually added a test with a message with fields deliberately out of order - I'm happy to do so though. It feels like it would make sense to be in google/src/protobuf, but it's not entirely clear what the rules of engagement are for adding new messages there. (unittest_proto3.proto?)

This is definitely not ready to ship - I'm "troubled" by the disconnect between a list of fields in declaration order, and a mapping of field accessors by field number/name. Discussion required, but I find that easier when we've got code to look at :)

Reflection refactor