Add response headers.

... Still need to react to requests.
Implement the HTTP cache validation shenanigans.
Force cache revalidation.
Coding standards.
Code standards, again.
Add checkbox to disable caching functionality.
Add test to test datastream cache responses.
Add missing function comment.
Fix some comments for Jordan.
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.