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.