feat(Windows): Adds Windows support to react-native-video

This PR adds react-native-windows support to react-native-video.  The Video component is implemented with a XAML MediaPlayerElement.  Most of the features implemented by Android (and some additional ones) are implemented by Windows.

Known issues and missing features include:
* onReadyForDisplay event
* local URI convention (e.g., "broadchurch" in examples changed to require("./broadchurch.mp4")
* `playableDuration` in `onVideoProgress` event is always 0.0
* `playInBackground` is not yet supported
* Volume settings are applied, but the UWP control does not handle it properly

windows/ReactNativeVideo/ReactVideoEventType.cs

@@ -0,0 +1,15 @@
+namespace ReactNativeVideo
+{
+    enum ReactVideoEventType
+    {
+        LoadStart,
+        Load,
+        Error,
+        Progress,
+        Seek,
+        End,
+        Stalled,
+        Resume,
+        ReadyForDisplay,
+    }
+}