Callback Extension - Updated

[ndbroadbent]

Hi Michael,

I've refactored and updated the callback extension, and have fixed the limitation I was mentioning before, where a hook cannot be modified multiple times. I'll let the commit message speak for itself:

• Heavily refactored and reworked callback extension. View hooks are now treated as a separate entity, distinct from controller and legacy view hooks. However, they both still use the same @@responder variable to store the classes that implement a given hook. View hooks are now stored as Procs in a hash instead of being defined as methods on the class. This allows a Callback subclass to chain data operations for the same hook.
• Improved documentation and tidied up code.
• Added specs for chaining view hook operations, created new CallbackSpecHelper module to contain helper classes and methods.

Thanks,
Nathan