Implement ajaxCache option

Allows for caching of upstream AJAX service responses per HTTP caching rules.
Fix ajax cache output
Drop clone operation from cached ajax response

It's reasonably safe to assume the the upstream implementation will return distinct objects that we can safely modify in downstream operations.
Include catbox memory package for tests
Fix catbox tests
Merge pull request #27 from walmartlabs/ajax-cache

Implement ajaxCache option