StreamingService: Don't assume that album, artist or title exist.

MPDroid's old behaviour when adding the album, artist or title to the
notification panel is if it's null the notification panel would print the
string 'null' to the notification panel.

The new behaviour checks for the field first, then is either omitted, as is the
case with album or artist, or is substituted with filename in the case of
the title tag.
StreamingService: Rename a couple of variables for readability.

Rename resumeStreaming() to beginStreaming() and oldStatus to prevMpdState
to improve the logical naming. They make more sense to me, I hope they do
for you as well.
StreamingService: Move all stubs to the end of the file.

Take the stubs out of the body of the code to increase readability.
StreamingService: Remove large nested blocks in stateChanged().

Removing large nested blocks improves readability.
StreamingService: Comment additions, cleanups and breathing space.

Add a few comments here and there, clean up some old comments and add some
space to increase readability.
StreamingService: Move coverart notification to it's own method.

Move the coverart stuff from showNotification() to it's own method. Also move
the MetadataEditor to the proper method, setMusicInfo(); this clears up a TODO
and should make things a little more clear for readability.