Change userid length to 64 in database schema

This is the length owncloud is shipping userids by default
move playlist-song relation table to the plural playlists_songs
make song_id field mandatory in a playlist-song relation
use text in the db schema instead of a timestamp

This change is needed to avoid problems with MDB2 failing when creating
tables with timestamps.
remove unnecessary comments from the subsonic lib
adds support for the owner parameter in getArtists

Its behavior is the same as the <id> parameter, no big deal.
adds a private call to get shared file paths
add query and exact params to getArtists call in collection_extra

This way it acts exactly as the one in collection, and at some point we
can just extend the collection lib.

Also, cleanup :)
add query and artist parameter to getAlbums in collection_extra
extend getSongs call with an added owner parameter
add owner parameter to search in subsonic

This works by using our collection_extra lib