Initial WPF Support (#385)

* initial support for WPF

* update readme with WPF specific instructions

* remove autogenerated .gitattributes file

* reference RNW NPM package instead of hard-coded local reference

windows/ReactNativeVideo.Net46/ReactVideoEventType.cs

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