Add FSharp style observable events for common cases

Why: This is ReactiveUI we love observables and C# events
suck. There is Observable.FromEventPattern but who remembers
the correct signiture for that. Not me! I just want to do this
in my code

    control.MouseDownObservable()
      .Sample(Timespan.FromMilliseconds(500))
      .Subscribe(x=>Console.WriteLine("Yeah!"));

This is easy in F# cause events implement Observable. We want
this in C#!.

How: T4 templates of course. At the top of the template you
put in some types and the template finds the associated assemblies
and then finds all the events for those assemblies and
then generates extension methods that wrap Observable.FromEventPattern
for you.

## TODO:

- [x] Split this into a separate library
- [x] Make it work with WinRT + WP8
- [x] Make some new NuSpec files, add the dependency to the ReactiveUI metapackage