Cross-Platform UI Project Templates using ReactiveUI

This PR drops a new project template based on [ReactiveUI](https://github.com/reactiveui/reactiveui), that helps you to create a cross-platform app. 

The idea here is that, the default template is this Uber-Project that adds templates for every platform (so in total, a File => New Project will create 6 separate csproj files - WPF, WinRT, WP8, Android, iOS, and the Shared ViewModel library). Then, the user can just shift-delete platforms they're not interested in building for. 

## TODO

- [ ] Make Core + WPF actually work
- [ ] Add in WinRT and Android
- [ ] Implement iOS and WP8 in [ProjectTemplates](https://github.com/reactiveui/projecttemplates)
- [ ] ...so that we can drop them in
- [ ] Fix up icons, make things Shiny™