fixe exception "too many values to unpack" on many to many fields

[thivolle-cazat-cedric]

Traceback (most recent call last):
 [...]
  File [...]/wtforms_sqlalchemy/fields.py", line 189, in get_pk_from_identity
ValueError: too many values to unpack (expected 2)

https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/orm/util.py#L225
get_pk_from_identity expect tuple with 2 elements, but, sqlalchemy identity_keyreturn tuple with 3 elements.

cls it is not used, so i only take the element 2.
I tried to run test, but 1 fails, on master too ERROR: test_convert_types (tests.tests.ModelFormTest)

running on my environment, it fix my trouble

[coveralls]

Coverage remained the same at 89.615% when pulling 50d08f2 on thivolle-cazat-cedric:feature-compat-identity_key into 6cfdbc4 on wtforms:master.

[ilpssun]

When will 0.2 be released?