Fix beet install.  Previously the dev branch of beets was installed in order to utilise some of the newer plugins.  However, as of June 6th 2013, stable version 1.2.0 has been released. Changed install line to download non dev release.

Enhancement : added discogs plugin now bundled with v1.2.0.  This is enabled by default and will query discogs for information during the tagging process.  There are now three methods beets uses during import/tagging: musicbranins, discogs and audio-fingerprinting.

Bump plugin version to 1.3