FrameworkHandling: support extracting Silverlight5.0 and NetCore4.5

It's small changes only, hope it doesn't clash with the recent work in the portable branch:

- Alias "5.0" to "50" (e.g. for Silverlight 5.0)
- Extract "netcore45" to Windows v8.0 ([pre-NuGet2.1 way](http://docs.nuget.org/docs/release-notes/nuget-2.1))
- Extract MonoAndroid/MonoTouch also if there is a v1.0 suffix present
- Add unit test to extract framework assembly references for MathNet.Numerics