Add convenience methods to DocumentRouter

[jklynch]

I propose some convenience methods for DocumentRouter along these lines:

• DocumentRouter.start_doc() return the start document or raise an Exception if the start document has not been seen yet
• DocumentRouter.descriptor_for_event(event_doc) return the descriptor document associated with the given event document from a dictionary of uid --> descriptor
• DocumentRouter.stream_name(event_doc) given an event document return its stream name by looking up the event's descriptor in a dictionary of uid --> descriptor

I find myself reimplementing this functionality virtually every time I subclass DocumentRouter.

[tacaswell]

I'm sold on the first two if we already keep enough state to answer that question, a bit skeptical but still net positive if we have to add the state tracking.

The third seems likes self.descriptor_for_event(event_doc)['name'] so neutral on it.

[jklynch]

addressed by #166