Models and their usage refactoring. Resolves #34.

 - Updated all the models so that they reflect actual scheme
 - Renamed several fields in the models to match Python conventions
 - Updated most of the code to use mongoengine ORM
 - Changed tags get_or_create in "next" view to atomic update_one as
   the former is deprecated and considered unsafe
 - Improvements to admin
Merge pull request #40 from dchaplinsky/feature/models_update

Models and their usage refactoring. Resolves #34.