IO-Support (fixed and rebased)

This is a rebased version of PR #81 by @airplanesrule
It includes the fixes from https://github.com/jrgnicho/universal_robot/tree/IO-Fixed provided by @jrgnicho
+ some additional fixes and restructuring to be more conform with the ur_driver package layout

It's tested on a UR5 v1.7!
Comments appreciated...
I wasn't able to build the until I removed the ur_msgs dependency from the find_package(...) list.  Including ur_msgs here causes catkin to try and build the ur_msgs package twice and errors out.