Remove aggregation

I'm not happy with how I've started to implement MongoDB's aggregation
framework. I've decided to back everything out and regroup.
Ignore Vim swap files
PEP257

Most of the docstrings are being brought up to the guidelines defined in
PEP257. There are some places where the suggested change just didn't
make sense and so the docstring was left alone.

There are also a few places where docstrings are being modified
slightly, usually to make them more consistent with docstrings for
similar classes or functions.
Correct terminology

All methods are functions, but not all functions are methods. Functions
that are not methods should be identified as such.