Add SDK-Robot version checking to RobotEnable

Adds version checking function in robot_enable.py and parameters
in settings.py to verify the Robot Software version matches the
currently checked out local SDK Software version.

* Examples and Tools will use the settings.py CHECK_VERSION
  variable to enforce a version check on all 'Rethink' programs
  by default. Setting it to False will override it.

Corresponding commits will be added to baxter_examples, and
baxter_tools.
Add Gripper version check

Enforces gripper version check on electric grippers so no one
bricks their grippers anymore.  Can be overriden in settings.py.
settings.py also holds the compatibility map.  Unmapped versions
will only warn (for future compatibility).
Updates per review #25

Import ordering.
Merge pull request #25 from RethinkRobotics/version_checks

Add SDK-Robot version checking to RobotEnable