Update API implementation

This re-implements VLCMLAlbumtrack as a feature of VLCMLMedia (as requested in #26 (closed)) and adds a method to delete a file from the file system using VLCMLFile running on a background thread to improve the responsiveness of the client app (as requested in #27 (closed)).

Additionally, this MR proposes various minor API additions that were previously missing compared to libmedialibrary. Finally, this fixes the implementation of the history() getter and returns the last modification date of a file as a NSDate instead of a time_t.