added IO capabilities

The IO modifications should be in the IO_testing branch.
we need to find an agreement with message gets which number
see https://github.com/ros-industrial/universal_robot/blob/hydro-devel/ur_driver/prog#L11
and https://github.com/ros-industrial/universal_robot/pull/78/files#diff-b2f4c5403df32a364117b89312e34ab7R11