refactored leveldb data storage

It needed levelDb objects to be used in a Singleton way, and each
instance shared a lot of behavior.  So I've revised Poke::Store to be a
base class which exposes a .store= method on each class to allow for
configuration (which is now done in each spec as needed, and all
together in the Poke module for the purposes of the app).
added rspec rake task (now default)