Update README.md for C#

Remove the C#-specific field_presence_test.proto, using unittest_no_field_presence.proto instead.

This is the start of establishing a C# namespace of "Google.ProtocolBuffers.TestProtos.Proto3" for proto3-syntax protos.
We could optionally split the directory structure as well into Proto2 and Proto3 for clarity.

C# generate_proto.sh and buildall.sh scripts

C# cleanup

Fixes for Mono

Convert back to using NUnit, which is now loaded via NuGet.

This includes the NUnit test adapter which allows NUnit tests to be run under VS without any extra plugins.
Unfortunate the compatibility tests using the abstract test fixture class show up as "external" tests, and aren't well presented - but they do run.

Performance optimization for small messages without unknown fields

Add C# namespace to .proto files (e.g. descriptors, unit tests)

Remove ClsCompliant declarations from C# code (fix #318)

Field Presence for Protocol Buffer C# Proto3

Change to using xUnit for all unit tests, and fetch that via NuGet.

This includes fetching the VS unit test runner package, so that tests can be run from Visual Studio's Test Explorer.

Convert C# projects to Portable Class Libraries