Work type handling

[boferri]

Add a property to relate work types to mo:MusicalWork instances. Currently, MBZ differentiate between 17 work types which are:

musicbrainz_db=# SELECT * FROM musicbrainz.work_type;
 id |    name    
----+------------
  1 | Aria
  2 | Ballet
  3 | Cantata
  4 | Concerto
  5 | Sonata
  6 | Suite
  7 | Madrigal
  8 | Mass
  9 | Motet
 10 | Opera
 11 | Oratorio
 12 | Overture
 13 | Partita
 14 | Quartet
 15 | Song-cycle
 16 | Symphony
 17 | Song
(17 rows)

This probably requires the creation of a new class for work types, e.g., mo:WorkType.