Added INetwork interface & async cache

Currently implements IsReachable for iOS, Android & WP8.

Added interface for async caching + SQLite caching implementation.
Needs unit tests.
Network and MediaPicker fixes

Added Network to iOS and Android devices. Registered Network and WP8
MediaPicker with Xamarin Forms DependenceResolver.
+++ fixes to Network & MediaPicker

&^&%%^& - fixing changes Git missed