process_url always returns a list
Update lookup() docstring
Small formatting and variable name changes

Some of these seem a bit more descriptive after going through the code,
though these really aren't important changes.
Inline dictionary comprehension and remove comment

The code seemed to be descriptive enough without the comment.
Tighten up percent change formatting and rename

Moved a lot of the formatting logic into the formatting string itself.
We found documentation here:
http://docs.python.org/2/library/string.html

We also renamed the function to more clearly indicate that a new key is
being added.