Symbolizing define_hooks parameters for Hooks gem

The API of the Hooks gem changed between 0.2.2 and 0.3.0. These changes
address those changes. It should be noted that Hooks 0.3.0 introduced
the concept of halting the callback chain when false is encountered.
As a warning, this update may create problems for other defined
callbacks.
Merge pull request #29 from jeremyf/updating-rubydora-to-latest-hooks-version

Symbolizing define_hooks parameters for Hooks gem