Added initial outline and interfaces.

This is the initial outline of the caching proposal, including the
interfaces themselves (which are the most fleshed out part of this
being added to the repo at the moment).
Added main  interfaces to their own files

This will allow people to actually use the interfaces.
Added the goal from the mailing list, as well as some formatting
Corrected some encoding errors
Added CacheIterator and CacheIterator
Added documentation to the CacheItem interface
Added additional comments to the CacheIterator, CacheFactory and CacheIteratorFactory interfaces.
Updated the format and added new sections

Changed some of the formatting around, added the "Multiple" section.
Continued expanding the documentation in the interfaces.
Changed the names of the various interface files to drop the leading (redundant) "Cache".
Continued with renaming by updating comments and classes.
Cleaned up formatting and whitespace.
Expanded the PSR-Cache proposal.

Extensively expanded the proposal. Added the new interfaces (with
comments), wrote out the introduction and data sections, and cleaned up
a lot of formatting.
Corrected formatting on the data list.
Updated the return value for Cache\Item->clear() function.

Clarified that the clear function should return true as long as the
item is not in the cache at the end of the call, regardless of whether
it was removed or never existed.
Added the Stackable and TaggableItem files.
Expanded the PSR

Clarified acceptable keys, clarified that null should explicitly be
null, altered header names, added text for namespaces, added the
TaggableItem interface, and removed the driver interface.
Merged the Factory and IteratorFactory classes into the Pool class.

By merging the two classes into one we create more of an environment
class, which makes certain extensions significantly easier to architect
simply.
Added the TaggablePool, StackablePool and StackableItem class. Removed the Stackable class.
Updated the PSR proposal to add Stacks and change formatting.