Remove memcache.

It does not exist in 3.0
Update docs to use 3.0 conventions.
Move creating a storage engine to the bottom.

This is less likely to be done and should be lower down on the page.
Add reference anchor.
Re-organize the cache documentation.

Use section headers to make docs easier to scan and add examples to
methods that lacked them.
Update docs and add xxMany methods.
Include examples of returns.
Merge pull request #1277 from cakephp/multi-cache

Update Cache docs.