Provide extension hooks in API class

I needed a way to extend and modify default Flask-Restless functionality and came up with this. Basically idea is such:

- Before-action hooks allow more complex permission check and modifying/augmenting request
- After-action hooks allow to modify response
- Any extension hook can abort request or return a completely custom response

I'm not yet entirely sure about API of these hooks. For instance with `_before_post` or `_after_get` is clear enough - pass the model about to be created in case of `POST` and pass model + its data in dict in case of `GET`.

With `_after_search`, `_before_patch`, `_after_patch` is a bit more tricky. Now I think best approach is to pass along as much data as possible and let extending class make sense out of it.

This also makes `post_form_preprocessor` from #74 obsolete.

Here's some code from my project in progress (and more examples are in tests): https://gist.github.com/3155303

This change would also allow to address #2 rather elegantly. E.g.: https://gist.github.com/3155322

What do you think?