Better support for tag names on dynamic objects, including id generation.

Right now, you can only change the raven-entity-name metadata after you have called store, and then it is too late for identity generation.  In order to fix this, you have to create overrides of the HiLoGenerator classes to include a call to your custom naming scheme, which seems heavy handed.  This change allows for people to override the entity naming scheme, generate ids based on that scheme, and if they don't override it will work just as it does today.