Update the ChangeLog
jshint <- happy
Check the 'responses' array in RoboHydraHeadStatic
Give proper 404 errors in the REST API

If the plugin, head or scenario involved doesn't exist, return a
proper, explanatory 404 error instead of dying with a 500.
Remove duplicated definition of startScenario (ugh!)
Priority dynamic heads now have precedence over admin plugin
Add reset method to heads, call it on scenario start

When heads have state (eg. RoboHydraHeadStatic when used with
multiple responses) and they are included in a scenario, they
need a way to go back to their initial state when the scenario
is started (otherwise, starting the scenario again won't be
reliable as the state of the head on scenario start won't
always be the same).

To solve this problem, a reset() method is added. By default
it does nothing, but for RoboHydraHeadStatic it makes it
start with the first response again.