Updated Firebase security rules for data model

Added support for indexes in the containers data type. Containers may
now include indexes of both their child containers and their contained
objects. Note that these indexes must be updated AFTER the element
they're pointing to is written.
Some Javascript for interacting with Firebase

This will eventually be included into a demo web app. For now, this
includes some bits on how to handle login (including creation of user
profile on first login) as well as a commented-out example of how to add
an object to a grocery list.