enttec openDMX

Great module!

I'm trying to get it to work with the [enttec openDmx](http://www.enttec.com/index.php?main_menu=Products&pn=70303), which should be very similar to your usb-dmx-pro driver.  I've determined that the baudrate is lower, but that's it so far.  I can connect to the device for a few seconds before it disconnects... so I've got something wrong with `send_universe`.  

Here's [a reference implementation in ruby](https://github.com/heisters/rdmx/blob/master/lib/dmx.rb) which I'm working from.

Any suggestions?