Clean up inline documentation and specs
Implemented BindingIndex strategy pair

The BindingIndex strategy pair for binding keys and invalidation captures the
spirit of the caching implementation of Garner 0.3.3 and earlier, while taking
advantage of the structural, consistency and performance improvements of
Garner 0.4.0+.
Tighten definition of BindingIndex.canonical? for consistency
Merge pull request #39 from fancyremarker/binding-index-strategies

BindingIndex strategy pair