django-fsm>=2 compatibility

## Backward compatible changes for django-fsm>=2

django-fsm>=2 requires django>=1.6. Using an older version of Django results in the error [TypeError: Error when calling the metaclass bases contribute_to_class() got an unexpected keyword argument 'virtual_only'](https://github.com/kmmbvnr/django-fsm/issues/30)

*Issue fixed*: Applying the **fsm_log_by** decorator prevents the transition from being returned by **get_available_FIELD_transitions** and **get_all_FIELD_transitions**. 

Changes:

* Imports now from **django_fsm** and not **django_fsm.db.fields** (affects tests)
* Added new tests to check transitions are returned from **get_available_FIELD_transitions** and **get_all_FIELD_transitions**
* Added **@wraps** to the **fsm_log_by** decorator to remedy the missing transitions

CI tests fail for where Django <= 1.6