Moved the DriverInterface class into the Stash\Interfaces namespace.
Added Item and Pool interfaces
Added Driver Extension Interfaces

These interefaces can be used to add driver level support for features
the backend system supports. This way behavior that is emulated in
Stash can be offloaded to that (probably more efficient) sysem.
Updated Pool and Item references to point at the Interfaces, updated comments.
Merge pull request #98 from tedivm/InterfaceRefactoring

Interface Refactoring