Added integer to control number of values returned by pylast's

get_top_tags. Integer is called maximum and is user definable in the
config file.

Some LastFM tags are meaningless. PyLast.get_top_tags occasionally returns wrong tags and if those tags are in your whitelist, they will be applied to the songs you are importing. So, I put another user definable variable called 'maximum' in the config file. This limits the number of genres returned by pylast.get_top_tags(). Thos tags are then compared to your whitelist. For example, before limiting get_top_tags, Al Di Meola was tagged as a metal, indie rock artist among other things, now his album 'Winter Nights', which is a holiday jazz guitar work, is tagged, Jazz, Guitar, Jazz Fusion, which I can live with. I set 'maximum' to 8.