Move artist property to Basic Object

upnp devices often return the artist property for the
`object.container.album.musicAlbum` class. By
restricting the property to the item level
(`object.item.audioItem.musicTrack`), it makes it awkward to
access this property from the music album folder level.

This patch moves the property up to the Basic Object level so it may be
used earlier.
Merge pull request #24 from squarefrog/master

Move artist property to Basic Object