Clean up the API extra data access hooks

Review Request #8084 — Created March 30, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x

Reviewers

The APIExtraDataAccessHook has been rewritten to utilize registries
for registration and unregistration of callbacks. Tests have been
updated for these changes.

The documetnation has also been updated and reformatted for clarity.

Ran unit tests.