Merge pull request #29 from ScotterC/association_cache

breaking up association cache methods
Merge pull request #30 from ScotterC/belongs-polymorphic-nil

Polymorphics don't necessarily have a type set.
Merge pull request #31 from ScotterC/return-nil-for-one-to-one

Return nil for one to one
Merge pull request #34 from ScotterC/memoization

Memoizing method and association cache methods.
Merge branch 'master' of github.com:flyerhzm/simple_cacheable into marshalling-tests