Document the SelectServer, Scheduler & Executor classes.
Remove the old form of SelectServer::RunOnce.

Introduce a non-blocking and a configurable version of RunOnce.
Merge branch 'master' of github.com:OpenLightingProject/ola
Fix Peter's comments on the SelectServer documentation.
Fix the tests by making RunOnce() do what it actually says.
Clean up the SelectServer documentation, add an example.
And another doc fix.
Change Remove*Descriptor to return void.
Fix a spelling mistake.
Start documenting the Descriptor classes.