Reorganized folders, added Rake-based build system, NuGet support

This pull request looks bigger than it really is.

I did the following things, in order to make it easier to consume Pigeon in client applications:

1. Reorganized the folder structure - all source code for the framework itself now goes under 'src', unit tests under 'test', etc...
2. Added a Rake-based build system, using [Albacore](https://github.com/Albacore/albacore) - includes the ability to automatically increment the major / minor / revision / build numbers of the binaries of the assemblies and NuGet packages, and includes the ability to create aforementioned NuGet packages. Includes all of the dependencies, including the Protobuff binaries.
3. Cleaned up some parts of the solution file - i.e. made all of the binary hint paths point to NuGet etc...

I set the initial version number for Pigeon to version 0.1.0.0 - if you're comfortable with this pull request I'll add a Github page explaining how to use the Rakefile to create new NuGet builds and bump the version numbers.

This build system is compatible with TeamCity and other common .NET build servers; we use it in production ourselves!