Support all the .NET platforms

Yo I heard you like .csproj files

## Ok for real what is this about

This PR adds support for Xamarin platforms and for Mono. To do this, we had to do some tweaking to the output directory layout - now project files are always named `TheNameOfTheProject-NugetTargetPlatformName.csproj` (i.e. `Octokit-MonoAndroid.csproj`), and the build output directories are now organized by target platform (i.e. it's not `bin\release`, it's `bin\release\NuGetTargetPlatformName`). 

## TODO:

- [x] Make sure building NuGet packages still work
- [x] Update documentation
- [x] Fix the test projects that somehow got messed up