Add response headers.

... Still need to react to requests.
Implement the HTTP cache validation shenanigans.
Force cache revalidation.
Coding standards.
Code standards, again.
Added the previously removed print callback, as it was required by another module and was breaking clipping.
Prevents a white screen when a viewer is set but not enabled.

If a user selects a viewer and then disables that module, but does
not change any solution packs viewers settings this can occur.
Fixed Menu path conflicts with printing dc_data and clipping large image and newspaper.
Removed console log function and reference to old view object permission.
Add checkbox to disable caching functionality.
Add test to test datastream cache responses.
Add missing function comment.
Fix some comments for Jordan.
Merge pull request #394 from MorganDawe/7.x

Added the previously removed print callback, as it was required by anoth...
Respond with the same headers as we did before, if caching is disabled.
Coding standards... Trailing space.
Merge pull request #393 from adam-vessey/7.x-datastream-cache-headers

Cache headers for datastreams.
Merge pull request #397 from nigelgbanks/7.x-solution-pack-viewer-whitescreen

Prevents a white screen when a viewer is set but not enabled.