#2 artist class ignores mbid

[Tim Jones]

In the Last.fm API spec, the Artist methods can be called with either the 'artist' parameter or the 'mbid' parameter. (The "musicbrainz id".)

However, this library's instantiation of those methods all fail if no 'artist' is provided, regardless of whether an 'mbid' is provided.

[Tim Jones]

Could be fixed by changing these lines
~~
if ( !empty($methodVars['artist']) ) {
~~

To instead read

~~
if ( !(empty($methodVars['artist']) && empty($methodVars['mbid'])) ) {
~~