Many changes:

- Removed the QueryManager class- all querying is now handled by the
Queryset class. This was necessary to manage chained query lines like

GameScore.Query.gte("score", 1000).lt("score",
2000).order("playerName")

- Queryset methods for comparison and options are now added
programatically (makes it easier to change their operation all at
once).

- Added test coverage of querying methods, including comparison
operators and limit/skip options.

- Removed user.py, moving its classes into `__init__.py`. The reason is
that lines like

import parse_rest
from parse_rest.user import User

parse_rest.APPLICATION_ID = "something"

meant the module attribute APPLICATION_ID was actually not accessible
from the User class. Perhaps there's a way to make this work- I just
couldn't find it yet.

- Set up createdAt and updatedAt to return date time objects rather
than strings