Added audio::SourceFile::getSupportedExtensions()

This can be used with `getOpenFilePath()` to gray out files that don't match the returned extensions vector.

This was straightforward on Mac, though my attempts to do it the 'right way' on Windows were in vein - I don't think Media Foundation provides any sort of api like this and at some point, it would take mapping the codec GUID's to string by hand.  So instead, I included all extensions from Microsoft's official [Supported Media Formats](http://msdn.microsoft.com/en-us/library/windows/desktop/dd757927(v=vs.85).aspx) page, which closely resembles the list provided on Mac by Core Audio.

Fixes #431