Raspberry Pi native UART DMX output plugin

Merge request for an output plug-in using a (suitable) hardware UART connected directly to the RS-485 line. This has been tested on the Raspberry Pi running Raspbian and works OK there. Other platforms and Linux distributions may vary. Three issues remain:
* The plug-in ID has not been assigned yet, so this code uses the testing ID.
* Some documentation on how to enable and use this plug-in is needed (where?), other than http://eastertrail.blogspot.co.uk/2014/04/command-and-control-ii.html
* There is an rare bug which reduces output to ~ 1 change per second. This is apparently random, if it occurs then all output is laggy/jumpy until the system is restarted. Not currently reproducible, no cause known yet.