PSR-6: Cache

Pull request for a Caching PSR.  This summary will be updated with the current status periodically.

## Status
The current text can be found here: 
https://github.com/Crell/fig-standards/blob/Cache/proposed/cache.md
https://github.com/Crell/fig-standards/blob/Cache/proposed/cache-meta.md

Please discuss any proposed changes on the mailing list before filing a pull request against this branch.

## 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.


Just to add to this, we have an "advanced" caching proposal that we've been pushing off until after the basic one has resolved. In that one we'll be able to throw in interfaces that only fit certain "types" of caches, so that features can be typehinted against through those interfaces and people can use some of the advanced functionality.