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...