Add Signaling to DigitalIO; New gripper-cuff control example

[rethink-rlinsalata]

Adding _changed Signal (& slot) to DigitalIO and accompanying new gripper-cuff ctrl example for DigitalIO Signaling

• Adds a state_changed dataflow.Signal() to the baxter_interface DigitalIO class, which allows callbacks to be connected easily.
• Also converts the state() into a property, which can be set/read directly.
• Demonstrates the added Signal feature of the DigitalIO class in an example that adds gripper open/close functionality to cuff button presses. This example can be run in the background as an easy means of controlling grippers while using the robot.

[rethink-kmaroney]

Look great - nice example. Small fixes. Make sure to run pep8 on updates.

[rethink-rlinsalata]

Pulling after making review changes...