Added support for embeded album art.

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.
Merge pull request #7 from rslomkow/master

Added support for flac embedded album art.