control default model table storage type from database configuration

At the moment, default model table storage type is hardcoded to `ROW` in [`creation.py` line 82](https://github.com/kapilratnani/django_hana/blob/master/django_hana/creation.py#L82) and each model using a different store needs to be decorated. This patch introduces a new database configuration key `DEFAULT_MODEL_STORE`, which can be used to specify a database-wide default store for all models. Moreover, the _default_ default model store (heh) is changed to `COLUMN`, as per this recommendation: http://scn.sap.com/community/hana-in-memory/blog/2013/12/29/6-golden-rules-for-new-sap-hana-developers