Added mongodb indexes

Resolves #13: Added MongoDB indexes for account.slug and scoped account
models for account_id, model_id. Elasticsearch is doing the heavy
lifting for object listings so extensive MongoDB indexing is
unnecessary.
Substitute 'today' with date string YYYY-MM-DD

Resolves #15: Quick and dirty query helper. It's unlikely that a user
will search for the keyword today. So we are replacing that keyword with
today's date as a string (2013-06-10). Useful in due date searches.

due:[* TO today] => due:[* TO 2013-06-10]
due:[today TO *] => due:[2013-06-10 TO *]