Adds the ability to restart experiments

This is essentially the inverse of Experimental::Experiment#end, which
we sometimes call in order to remove experiments from production that
aren't quite ready for prime time after they've been deployed (for
whatever reason).

Adding this method so we have a single way to restart those false starts
Better test for when the experiment is not ended
Merge pull request #19 from howaboutwe/adds_experiment_restart_capability

Adds the ability to restart experiments