[WIP] Add callbacks

I would like to introduce callbacks as a mean to perform actions before `update`, `insert` and `delete`.

Here is a first draft I am working on as a basis for discussion.

It is my hope that if I get this right, we can add:

- Serialisation (maybe that would be a good way to tackle hstore, json, ... data types)
- Timestamps
- Automatic validations?

ToDo:
- [x] Documentation
- [ ] Validations
- [ ] Add params to functions
- [x] Reconsider naming (CRUD vs. Repo methods)
- [x] Hook up callbacks to Repo (actually invoke them)
Actually, we don't need __callbacks__ at all. Define a @before_compile callback that will compile all events (before_insert and so on) into actual functions in the model. So in order to call the before_insert callbacks for MyModel, we just need to do `MyModel.before_insert(model)`. This will allow us to use local private functions and it will be the fastest impl possible.