Adds gripper convenience methods for resetting custom gripper properties and state.
Latches/verifies gripper state/properties setters

Latches reset_custom_properties, reset_custom_state published setters.
Adds wait_for to verify state/properties reset succeeds.
Updates per review #28

reset_custom_<state/properties> uses slots and slot_types for attribute modification.
gripper reset_custom_<state/properties> docstring formatting
Merge pull request #28 from RethinkRobotics/reset_custom_gripper

Adds custom gripper reset convenience methods