Disk-only cache writing

Adding selectors to allow disk-only cache insertion; this is handy for pre-caching objects from network responses that may not be immediately used.

This includes edits to update the comments on the 'setObject:' selectors for indicating the asynchronous nature of writing to the disk (as opposed to the synchronous insertion into the memory cache), along with an overloaded selector for specifying a callback block to listen for the completion of disk writing operations.