Add the GPIO plugin.

I need to do some timing tests.
Merge branch 'master' of github.com:OpenLightingProject/ola into gpio
Merge branch 'opc' of github.com:nomis52/ola into gpio

Conflicts:
	common/protocol/Ola.proto
Add the missing Makefile
Minor fixes to the GPIO plugin.
Merge branch 'gpio' of https://github.com/nomis52/ola into opc
Doxygen fixes