Chrome performance: cache potentiallyApplicableRulesets

(for issue #12)

The function "potentiallyApplicableRulesets" is hot, and its results are stable.

From profiling, this is *very* slow, and it's easy to see why. The calls look like:

     Loading www.nytimes.com ->
          Load every image (20+ images at graphics8.nytimes.com)
	       Call potentiallyApplicableRulesets every time
	       	    Return the same result every time :(

Let's use a simple LRU cache to store those results.

This adds the MIT-licensed LRU cache from: https://github.com/rsms/js-lru

The cache isn't persistent (it's in memory, not LocalStorage or elsewhere),
so we don't have to worry about stale data when the extension updates.

Signed-off-by: Nick Semenkovich <semenko@alum.mit.edu>