Add comments to the compatibility module

Some of these probably aren't necessary, but they'll help to keep the
module organized should more things be added to it.
Use the correct iterator for dictionary keys

The `iterkeys()` function in the compatibility module is intended to be
used as a substitute for `.keys()` and `.iterkeys()`. As part of the
transition to Python 3, `iterkeys()` should be used instead of these
methods to make sure an iterator is always returned.

Additionally, the `get_next()` function can be used to allow proper
iteration over these iterators. In instances where only the first key is
needed, using `get_next()` is a better approach that using `list()` and
consuming the entire iterator.

References #23