Added Drive and Turn Commands and Tracking for Charger

[reakain]

Added some of the functionality from Issue #24

Added properties, events, and event handlers for tracking:

• is_charging
• is_on_charger

Added Methods:

• drive_off_charger_contacts()
• drive_straight()
• turn_in_place()
• turn_in_place_at_speed()

Drive off charger contacts is a little weird. The go to pose function call got more consistent results for getting off the charger, and doing so required disabling and then re-enabling the cliff detection. It also currently does not check that it's even on the charger before completing the command.

I'm not sure how well I followed the contributing guidelines, but I have been running these changes on automatic multi-month Cozmo systems for about 8 months, so I know they at least work in the wild.