Add schema to msgpack branch.

Features:
    * You may use space names instead of id's.
    * You may use index names instead of id's
    * Getting schema on demand.
    * If space/index doesn't exists on first demand -
      it'll throw SchemaError(child of DatabaseError)
    * You may flush Schema from time to time, if your
      tarantool's schema may alter.

Also a little of pylint'ing.