Added support for flac embedded album art.

Basic idea is that if no album art is present on the filesystem check the flac file for embedded album art and stick in a temporary file for processing.

I have only tested on Linux (Ubuntu 14.04)
did test.

1) No tempdir
2) Pre-existing tempdir
3) tempdir with wrong permissions
4) pre-existing temporary artwork file
5) non-existing temporary artwork file
6) flac file with no embedded artwork (produces an error message) but works.

KNOWN ISSUES:

does not automatically clean up temporary files (but are clearly marked and in system temp directory)
does add more resource usage in the initial scan phase as it does a check of every file and build an md5sum
only checks security of temp directory on posix systems
creates more warning when no embedded album art is present