Add group support for proto2

* Compiler changes

* Generated code changes

* Library changes

* Compiler style changes

* Generated style changes

* Fix Windows build errors

* Implement changes from review

* Reintroduce proto2 check

* Compiler changes (required handling review)

* Generated code changes (required handling review)

* Library changes (required handling review

* Field presence rewrite (compiler changes)

* Field presence rewrite (generated code changes)

* Compiler comment

* IFieldAccessor.HasValue library implementation

* Remove Clear methods and default values from proto3 code (Compiler)

* Remove Clear methods and default values from proto3 code (Generated)

* Remove Clear methods and default values from proto3 code (Library)

* Fix distcheck error

* Rewrite default string values to use base64 and convert

* Library changes (IMessage2)

* Compiler changes (IMessage2)

* Generated changes (IMessage2)

* Rebased and regenerated

* Compiler changes (initialized extension)

* Generated changes (initialized extension)

* Library changes (initialized extension)

* Refactor MessageExtensions.IsRequired

* Move string default value creator and bytes default value creator back to seperate methods

* Dead code cleanup

* Fixed segmentation fault
Removed unused header method declarations

* Edited MessageFieldGenerator to clone to backing field instead of property

* Generated C# proto code

Add preserve UnknownFields
Compare floating point values bitwise
Add auto-generated header to C# generated files

* Add auto-generated header to C# generated files

* regenerate C# protos

 Add unknown field support for csharp

JSON tests fail, as we're not using OriginalNameAttribute yet.

Generated code changes for previous commit (basically InternalBuildGeneratedFileFrom => FromGeneratedCode)

Other changes are due to the well-known types changing without us regenerating.

Generated code changes from previous commit

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.

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

- Remove some old proto2-based C#-only messages
- Remove the "build" directory which only contained out-of-date files
- Remove the csharp_namespace option from proto2 messages
- Change "Google.ProtocolBuffers" to "Google.Protobuf" in other messages

(Primarily this is starting the hash code of messages at a non-zero value...)

Fixes issue #523.