Add optional enabled flag to inquisitions.

This flag is currently not respected by any inquisition code, but that should likely change in the future. For now, it exists to allow local code to easily enable and disable inquisitions if necessary.
Add enabled flag to the inquisition edit page in the admin.

By default it is hidden, as subclasses are responsible for respecting the flag. As well, the code path to save the flag checks to make sure the widget exists before attempting to save it, to ensure it doesn't break existing subclasses.
Clearer comment explaining the enabled property.
Add translatable
Add comment to the xml explaining why the enabled checkbox is hidden by default.
Reorder statements to keep the values array beside its usage.
Better comments from Nick.
Merge pull request #24 from isaacgrant/enabled-flag

Add optional enabled flag to inquisitions.