Bug 824099 - don't crash and ignore m4v movies and trash in the music metadata parser r=dkuo a=blocking+

This pull request fixes the crash in bug 824099 and makes a few related improvements to music metadata parsing.  It now starts off reading a 64kb chunk of the music file. If metadata is at the start (as it usually is) we should get everything in that chunk and not have to do another async read.  We now also explicitly ignore .m4v files.  And mediadb now ignores any files in hidden directories whose names begin with .).