First pass: cache manager uses SmartStore

Not sure that this is ready to merge into unstable yet, as it's all untested code.  It builds though. :)  This is a good cutoff for initial review.  Updates here are switching cache manager off of the file system, and onto SmartStore as the backing persisted cache.
Read and write methods all deal with / expect / validate some JSON-serializable type of data that's relevant to SmartStore: either an `NSDictionary`, or an `NSArray` of `NSDictionary` entries.