Merge pull request #1067 from davidpersson/cache-scoping

Implementing cache key scoping.
Expanding docblocks for cache clear methods.
Return false if cache file adapter clear method fails.
Prevent file cache adapter clear method from going recursive.
Implementing GC for cache file adapter.
Documenting options on cache methods.
Updating docblock for cache clean method.
Fixing formatting in docblocks for respondsTo.

Slightly refactoring Redis::respondsTo().
Delegate method calls to memcached connection object.

Adding tests.
Updating delegation docs in redis adapter.