Code review comment for lp:~thekorn/zeitgeist/flexible.engine.backend

This branch implements the flexible engine framework for zeitgeist, as discussed on IRC recently.
The most important changes are:
  * get_default_engine() is now in _zeitgeist.engine instead of _zeitgeist.engine.engine
  * get_default_engine() either returns the running engine instance, or creates a new one. To get the type of the new engine it looks in the ZEITGEIST_ENGINE environment variable, if not set it uses a hardcoded default, which is storm for now
  * new engine implementations can be added by creating _zeitgeist/engine/SOMENAME_engine.py, where SOMENAME is the type of the engine (like sqlite3, sqlite3_querymancer, couchdb etc.)
  * all engine tests are running using the strom engine implementation