Record operations only for signalling sessions.

In some scenarios model classes derived from SQLAlchemy(...).Model
may be used with basic SQLAlchemy session instances which aren't
extended with signalling functionality.

Before this fix, when you try using a vanilla session with
such models, you would get exception like this:

  File ".../flask_sqlalchemy.py", line 163, in after_insert
    return self._record(mapper, instance, 'insert')
  File ".../flask_sqlalchemy.py", line 172, in _record
    s._model_changes[pk] = (model, operation)
AttributeError: 'SessionMaker' object has no attribute '_model_changes'

With this fix, this problem is resolved.
Merge pull request #109 from paxan/bugfix/record-ops-only-for-signalling-sessions

Record operations only for signalling sessions