Add nested method to help with document mapping

cad46aa introduced a much cleaner update system. There are a couple of places the code could have been a bit cleaner, though.

The nested `map_field_names_and_values()` method addressed one of those. `_update()` can now handle mapping field names and getting values from the internal document with a single line of code (or two in the case of an atomic update).
Typo
Clean up a test's docstring
Add whitespace to docstring
Refactor database reads

This update is much in the same vein as cad46aa.

Adding the `_find()` method to the `Model` class will allow for more consistent changes to be made to `find()` and `get()`.

An important change is being made to the API of `find()` and `get()`. The `qs` argument is being deprecated in favor of the new `q` argument. Both arguments will be supported for the time being, but using `qs` will result in a `DeprecationWarning`.