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...
as there are no build_depends defined in the package.xml there should not be any components listed in find_package anyway....and we do not have build_depends as it is a python package...