Added SoundEffect and Song support to WinRT

Apparently, it was already possible to play mp3 Song on WinRT, but the code in the ContentManager was excluded by a #if !WINRT .

I also added .wma as a supported extension for Song on WinRT.

I then added the SoundEffect(string filename) constructor based on the example given here : http://sharpdx.org/forum/7-documentation/39-audio-samples

Finaly, I resolved a NullReferenceException in the static constructor of the MediaPlayer with the help of this : http://stackoverflow.com/a/17315187