Current task list (not in order)

- Optionally remove dependencies to csharp options
- Remove multifile support
- Mono support
- Docs
- Clean up protogen code
- Add flags to protogen
- Avoid using reflection for messages which don't need it (is this
  possible?)
- Add RegisterAllExtensions
- Add ToBuilder changes from Google's r92
- Silverlight changes (as per email to Jon)