Adds optional angular threshold argument for move_to_joint_positions in limb.py.

Optional argument allowing the setting of angular success threhold for joint position moves using move_to_joint_positions in limb.py. Default value is the same 0.008726646 (0.5 degrees) from settings.py. This option allows for fine/gross position settings of joint motions.
move_to_joint_positions no longer raises an exception on failure. It will now simply report its timeout as raising was overkill for this method.
Merge pull request #18 from RethinkRobotics/limb_move_to_threshold

Adds limb move_to_joint_positions optional argument - threshold