[artwork] do not add cache entry if reading artwork failed
Merge pull request #66 from chme/artwork3

[artwork] do not add cache entry if reading artwork failed
Avoid threading issue in both ffmpeg and libav that prevents decoding embedded png's
Updated artwork section in the readme
Merge pull request #67 from chme/artwork4

Updated artwork section in the readme
Lower log level for missing revision-number in client update request.  This is some clients' normal behavior, so we shouldn't need to need to see a message about it in the error log.
Fix bad free when encountering an error fetching query results.
Fix invalid read on empty string queries.
Merge pull request #70 from jmaggard10/fixes-20141217

Fixes misc bugs
improve selects for files table by removing the join to the groups table
db update v16 and cache update v2
Initial support for mpd protocol