Cache

Pull request for a Caching PSR.  This summary will be updated with the current status periodically.

## Status
 As of this writing, this text is identical to that previously in a pull request from tedivm.  Minor tweak pull requests will be accepted against the Crell/Cache branch at this time, but not major ones.

## Next steps
[ ] @Crell will review the text and tighten it up to make it more Spec-Precise and detailed.  Expected completion: ~July 2nd.
[ ] @Crell will start a series of mailing lists threads to discuss specific portions of the spec.  These will be focused and time-boxed.  Expected start: ~July 3rd.

## To participate
* Please *do not* discuss details of this PR as a PR!  I know it's weird, but that's the way FIG works these days. :-)
* To discuss a specific section, comment on the FIG mailing list in the thread specifically for that section.
* To request a specific change based on discussion on the mailing list, open a pull request against the Crell/Cache branch from your own fork.  Please do not open PRs that have not already been discussed at least briefly on the list.