Comparing databases with help of SQL queries instead of sets comparison:

* Only primary key difference (last_fm_id, artist) is taken in account,
  no changes if date, city or country are changed
Reworked config_parse function, now not all options are required.
additional_artists option in the configuration file is optional, so can be omitted
Performance optimization: using union instead of slow .add() for library and additional artists
Fixed template markup, now it's totally valid