Don't instantiate classes

This (a) may speed things up a bit, and (b) allows the detector to be used on objects that require arguments for construction.
Add el cheapo memoization

Trying this gem on a 500-line ActiveRecord::Base subclass generates quite a bit of waste heat.  :)
Fix indentation

(Can be squashed with previous commit, but I wanted to make the actual changes as obvious as possible.)
Merge pull request #2 from geeksam/dont-instantiate-classes

Don't instantiate classes
Merge pull request #3 from geeksam/memoize

Memoize