Fix gripper type signal w/ui_type check and init

Fixes bug in the on_changed signal for gripper type where msg
ui_type (numeric) was being compared against type() (string) so
signal was firing constantly.
Re trac bug: 10006

Also uses invalid initial value for properties to make sure signal
is not triggered on first message received.
Merge pull request #27 from RethinkRobotics/fix_gripper_signal

Fix gripper type signal w/ui_type check and init