Additional info on state changes of buttons

[sven-bock]

DigitalIO and Navigators

This little improvement enables you to see which buttons was pressed in your callback function.
Therefore you need only one callback function for all your buttons.
e.g.:

from baxter_interface.digital_io import DigitalIO
def updateState(*args):
    print "button pressed", args
button = DigitalIO("left_shoulder_button")
button.state_changed.connect(updateState)    

Camera

The current implementation raises an exception, if the camera is currently closed. I think it is useful in the camera controller to check, if the camera is opened or closed. Also in the past the service call to the camera list yielded all 3 cameras, which is not the case any more. Therefore I recommend to change the condition of the if in the init

[rethink-imcmahon]

Great pull request @sven-bock! I'll review it and get back to you. In the meantime could you rebase and PR into development? Thanks!

[rethink-imcmahon]

superseded by #62