Fixed an oddly stated error message.

The permissions to update and delete a survey response are the same, so they were using the same code. However, the error message only referenced deletion. It has been updated to state that the survey responses cannot be "modified" instead of "deleted".

References #491
Removed debugging messages.
Fixed a bug regarding storing observer definitions.

There was a bug regarding how optional metadata field definitions were being saved. This has been fixed.

Also, some unnecessary debug messages have been removed.

References #491
References #486
Merge branch 'ohmage-2.14' of github.com:cens/ohmageServer into ohmage-2.14