Support embedded cover art (#48)
Create a new temp cover file for each track

If we use the same temp cover file the mediaplayer g-s extension won't change
the cover because is it always the same path
Store current song metadata in MPDWrapper

- Update and format the metadata only when needed in MPDWrapper
- MPRISInterface returns MPDWrapper metadata on __get_metadata calls
Merge pull request #49 from eonpatapon/embedded-art

Support embedded cover art (#48)