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/.gitignore

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+#Visual Studio files
+*.[Oo]bj
+*.user
+*.aps
+*.pch
+*.vspscc
+*.vssscc
+*_i.c
+*_p.c
+*.ncb
+*.suo
+*.tlb
+*.tlh
+*.tlog
+*.bak
+*.[Cc]ache
+*.ilk
+*.log
+*.lib
+*.sbr
+*.sdf
+*.opensdf
+*.opendb
+*.unsuccessfulbuild
+*.lastbuildstate
+ipch/
+[Oo]bj/
+[Bb]in
+[Dd]ebug*/
+[Rr]elease*/
+*.tlog/
+Ankh.NoLoad
+UpgradeLog.htm
+
+#MonoDevelop
+*.pidb
+*.userprefs
+
+#Tooling
+_ReSharper*/
+*.resharper
+[Tt]est[Rr]esult*
+*.sass-cache
+
+#Project files
+[Bb]uild/
+
+#Subversion files
+.svn
+
+# Office Temp Files
+~$*
+
+# vim Temp Files
+*~
+
+#NuGet
+packages/
+*.nupkg
+
+#ncrunch
+*ncrunch*
+*crunch*.local.xml
+
+# visual studio database projects
+*.dbmdl
+
+#Test files
+*.testsettings
+
+#Other files
+*.DotSettings
+.vs/
+.vscode/
+*project.lock.json
+
+#JavaScript files
+*.jsbundle
+
+# Visual C++ cache files
+ipch/
+*.aps
+*.ncb
+*.opendb
+*.opensdf
+*.sdf
+*.cachefile
+*.VC.db
+*.VC.VC.opendb
+
+# Build results
+[Dd]ebugPublic/
+[Rr]eleases/
+x64/
+x86/
+bld/
+[Ll]og/