Set workflow_id up properly in RunsController#new.

This wasn't being set before being passed on, which is OK for a web form, but
breaks the flow for JSON, etc.

Views also updated to reflect this change.
Move new run parameter initialization to a before filter.

This gives us a proper skinny controller.
[TAV-377] Output JSON for the runs controller.

Now get responses from new and create.
Use responds_to/responds_with in runs controller.

Much more "rails 3"; much cleaner for the different responses required by html
and json.
Use responds_to/responds_with in service credentials controller.

More "rails 3" and a lot simpler. Also minor fixes in associated tests.
Move the decision to destroy a run, into the run model.

The before_destroy callback is used to determine whether to kill the run or not,
based on its status.
[TAV-402] Enable changing a run's name.

Add the update route to the runs controller but filter the parameters so that
*only* the name can be changed for now. This could probably be done better by
using Strong Parameters (Rails 4, or via the String Parameters gem).