Merge pull request #83 from mrbrl/xamarin_master

ViewModel constructor flexibility
fixing bug when no DI container was specified
Renaming to WP8 to mantain coherence with Xamarin.Forms closes #60
Mark RelayCommand as deprecated. use Xamarin.Forms.Command closes #72
added missing await on samples closes #70
Fix ViewFactory initializer and caching issues
removed  ViewFactory.CreatePage(Type type) as it was unnecessarily forcing the initialiser Action<TViewModel, Page> to always be the base class ViewModel
Allow cached page to be updated by initializer (coders can provide their own logic down the line to keep the page content static)
throw exception when viewmodel cannot resolve (educational)