RxUI 5 backports

This branch brings some of RxUI 5 features to RxUI 4.x without creating any breaking changes to RxUI 4.x.

Included are:

- Fix for #278. 
- ToProperty overload with out parameter.
- `ScheduledSubject` with a custom `ISubject<T>`.
- ListenIncludeLatest on `MessageBus`
- Ability to specify a converter per binding next to global.
- Fix variance on WP7 for `IMoveInfo<T>`

Most of the changes are additions or extra optional parameters. These changes should not break any existing functionality, but they will make RxUI 4.x more compatible with RxUI 5, which is very useful in the case where you have a Windows Phone 7.x and a Windows Phone 8 application.