Allow license URLs to be customised by calling code.

Currently the license acquisition URLs for protected content (e.g. PlayReady) are baked into the media files, and there isn't a way to change this at run-time. Have added a way for the client code to set an (optional) resolver for these URLs based on the keySystem identifier (e.g. com.microsoft.playready)
Add test for MediaPlayer for setting license URL resolver
Merge pull request #1 from jonoblinkbox/ConfigurableLicenseUrls

Allow license URLs to be customised by calling code.