Use albumartist in DatabaseCommand_AddFiles

So far, the album database uses the track artist. This changes it to use the album artist if one is available.
Merge remote-tracking branch 'upstream/master'
Check whether albumartist is blank before trying to add it.
Merge branch 'master' of https://github.com/hansschmucker/tomahawk
Only add albumartist if it's non-empty
Merge pull request #273 from hansschmucker/master

Use albumartist in DatabaseCommand_AddFiles