Port callback functions into hookset

[paltman]

Closes #106

[ossanna16]

@brosner Can this be merged?

[brosner]

This PR raises the question of compatibility. This change is backward incompatible. Do we accept it and release 2.0? If so, what about support for 1.X? I suppose the answer to the latter question is depdent on the support of 16.04.

It would be possible to support both and release 1.4. Have the hookset call the old callbacks.

Thoughts?

[jtauber]

I propose going ahead with this as @paltman proposed, with the backwards incompatibility. It's cleaner plus it gives us an opportunity to decide in 16.07 some of our policies and processes around BIs and how we document migrations between distribution releases, etc. This is a nice simple case to prove that out on.