Csharp update

First steps to making the C# runtime work with new codegen

(Having regenerated descriptor.proto relative to src, the earlier commented-out code checking that dependencies match may now be okay to uncomment again. Will experiment in later CLs.)

This commit includes changes to the C#-specific protos, and rebuilt versions of the "stock" protos.
The stock protos have been locally updated to have a specific C# namespace, but this is expected to change soon, so hasn't been committed.
Four areas are currently not tested:
1) Serialization - we may restore this at some point, possibly optionally.
2) Services - currently nothing is generated for this; will need to see how it interacts with GRPC
3) Fields beginning with _{digit} - see https://github.com/google/protobuf/issues/308
4) Fields with names which conflict with the declaring type in nasty ways - see https://github.com/google/protobuf/issues/309

1) Remove CSharpOptions
2) A new version of DescriptorProtoFile (with manual changes from codegen - it would otherwise be Descriptor.cs)
3) Turn off CLS compliance (which we'll remove from the codebase entirely; I don't think it's actually relevant these days)
4) Add "public imports" to FileDescriptor, with code broadly copied from the Java codebase.
Lots more changes to commit before it will build and tests run, but one step at a time...

Add C# generator files to the VS project for libprotoc

Also ignore Windows native build output files.

C# protocol buffers (initial pull request)

Rewrite of C# protogen to C++ (initial version)

Pulling protobuf-csharp-port into csharp/ directory

This is necessary for testing on iOS.

Change-Id: I54ec1e3aa2e9fbfff9a6cd6580920a6a62069b63

Fix for issue 283: JSON handler cleanup typo causing segfault.

Includes repro test case from @wfarr.