More alignment of data structures to DIDL

MS data structures moved to their own module. Unnecessary structures removed. Doc fixes
Adding missing file!
Support for radio stations
Fix unicode bug
Album art URI fix
Remove redundant code
pep8
Revising docs for new data_structures
Renaming all ML* classes to Didl*

--HG--
extra : amend_source : 293245673c1db9c03cc1cf95c67d7851a3bce937
pep8/pylint
get_ml_item renamed to get_didl_object in line with other renames
Code review fixes
Merge pull request #243 from lawrenceakka/align_data_structures

More alignment of data structures to DIDL