Adding USB Modbus support

Adding comModbusUsb, which implements the USB Modbus protocol.
Identical interface is maintained.
Adding robotiq_c_ctrl

Adding a support class for better interacting with the C model
control classes.  Provides easier to understand functionality and
commands
Corrected tcp/usb switch
Changing filenames from Usb to Rtu
Splitting robotiq_modbus_tcp into rtu/tcp versions.
Merge pull request #5 from gt-ros-pkg/groovy-devel

Adding USB Modbus support and RobotiqCGripper