Viewers can be defined by content type as well, for aggregate objects such as books.
Merge branch '7.x' into 7.x-allow-viewers-based-on-content-type
Merge branch '7.x' into 7.x-allow-viewers-based-on-content-type
Updated function header.
Parameters documented for viewer functions.
Merge pull request #230 from nigelgbanks/7.x-allow-viewers-based-on-content-type

7.x allow viewers based on content type