Welcome Back Microsoft.*

I want :boom: all the .csproj files we have floating around for the different platforms we're targeting.

Previously, we were stuck with platform restrictions on the Microsoft.* projects. But now we're totally :cool: with [the recent changes](blogs.msdn.com/b/dotnet/archive/2013/11/13/pcl-and-net-nuget-libraries-are-now-enabled-for-xamarin.aspx) we're now free to target non-Windows platforms :dancers: 

So let's do this gradually:

 - [x] introduce Octokit.Portable project, targeting Profile78
 - [ ] investigate build errors
 - [ ] update build scripts to make Octokit.Portable the source of truth  
 - [ ] update Octokit.Reactive to reference Octokit.Portable
 - [ ] deprecate Octokit.Mono* projects

Open Questions:

 - @paulcbetts you mentioned some issue with Rx and Mono on OS X. Any pointers/resources about chasing this up?
 - do we want to target .NET4? I was planning to leave the main Octokit project for that rather than deal with all the headaches of using a different profile.