A number of changes:
* Added exported functions to close connections and sessions.
* Added exported functions to create and drop tables. Currently not
happy with the shape of these functions, though, as they treat
tables in Erlang just as strings, and they force users of sessions
to have to pass both sessions and tables into get/put/delete
operations. It might be better to take a table identifier as part of
the session creation function and store that identifier with the
session resource.
* Added implementations for key-value get, put, and delete.
* Added very minimal unit tests for the changes above.
