Implement filter on ManyToMany relationships

[ged]

should simply add criteria to secondaryjoin

[ged]

This is not as easy as I thought: you have to reuse the same table, and since a ManyToMany generated table (eg the one of the "unfiltered relationship") is not available at definition time, it fails...

class User(Entity):
    name = Field(String(50))
    addresses = ManyToMany('Address')
    boston_addresses = ManyToMany('Address', table=addresses.table,
                                  filter=lambda c: c.city == u'Boston')

Supporting a callable table argument might be a solution, though that'd be kinda ugly.