Welcome Back Microsoft.*

This should fix #218 but let's see how much :rage1: is involved

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](https://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: 

There's some headaches with using the Xamarin PCLs at this point, so I won't obsolete them just yet.

- [x] introduce PCL (Profile78) and .NET4 builds for Octokit and Octokit.Reactive
- [ ] get PCL builds running on Janky cc @half-ogre 
- [x] fix impacted tests
- [x] investigate whether we can make ConcurrentDictionary redundant
- [x] update the paths for the NuGet packages to use the right profiles
- [x] get FAKE to support dependency targets in nuspec or drop System.Collections.Immutable usages

Test Scenarios:

 - [ ] install new package into .NET4 project, use code snippet to delete some repos from test account
 - [ ] install new package into .NET45 project, use code snippet to delete some repos from test account
 - [ ] update package into a .NET45 project, use code snippet to delete some repos from test account
 - [ ] update package into a Windows Store Application, use code snippet to delete some repos from test account