Add commit_many feature

[RoundelRider]

Adds a commit_many() function to the BaseDataProxy and framework specific DocumentImplementation objects.

This enables single operations to change multiple documents, for example:

query = {'inventory_type': 'widget'}
WidgetDocument(reorder=True).commit_many(query)

This allows schema validation of the objects fields, and produces the correct payload for the pymongo update_many() function.

See #384 for a more detailed description.