SQLAlchemy Adapter

Work in progress, see #277.

Notes:

• When receiving an sqlalchemy orm instance back from pyamf and you want to save the update, then you need to merge the object back in to the session. This is done by:

        session.merge(my_instance)
  

  see: http://www.sqlalchemy.org/docs/05/session.html#unitofwork_using_merging

• Always add all SqlAlchemy? ORM classes to pyamf using register_class. Otherwise the following exception can occur:

  Traceback (most recent call last):
    File "test_sqlalchemy.py", line 83, in test_sa_lazy_load
      user = decoder.readElement()
    File "pyamf/__init__.py", line 485, in readElement
      return func()
    File "pyamf/amf3.py", line 1125, in readObject
      setattr(obj, k, v)
    File "sqlalchemy/orm/attributes.py", line 116, in __set__
      self.impl.set(instance_state(instance), value, None)
    File "sqlalchemy/orm/attributes.py", line 619, in set
      lambda adapter, i: adapter.adapt_like_to_iterable(i))
    File "sqlalchemy/orm/attributes.py", line 652, in _set_iterable
      collections.bulk_replace(new_values, old_collection, new_collection)
    File "sqlalchemy/orm/collections.py", line 643, in bulk_replace
      constants = idset(existing_adapter or ()).intersection(values or ())
    File "sqlalchemy/util.py", line 978, in intersection
      self._working_set(self._members.iteritems()).intersection(_iter_id(iterable)))
  TypeError: dict objects are unhashable