Expose memoized function's memos

Add a `memo` property to the return value of `async.memoize` (the
memoized function) to expose the collection of already-calculated
values.
Add documentation for exposed memoized memos (3405799)
Add test for exposed memoized memos (3405799)
Merge pull request #175 from parshap/feature/expose-memo

Expose memoized function's results cache