WIP: Migrating to using Portable Class Libraries

I initially had grand plans to have a .NET 4 build alongside this, but it was much :rage1: so that's been delayed...

- [x] switch the Octokit and Octokit.Reactive projects to target Profile78
- [x] introduce Microsoft.Bcl.Async and Microsoft.Net.Http dependencies
- [x] resolve impacted test
- [x] test Xamarin Studio (stable channel?) can build the solution still
- [x] update `CreatePackages` step to ensure paths are correct and it all Just Works
- [x] ensure build server can compile and test this project before merging cc @half-ogre 
- [x] update docs to indicate that VS2013 is now necessary
- [ ] update dependencies to ensure they include the necessary Async/Http/Immutable libraries
- [ ] test the packages install for .NET45 and Win8 because paranoid