[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
- [x] ~~Validations~~
- [x] ~~Add params to functions~~
- [x] Reconsider naming (CRUD vs. Repo methods)
- [x] Hook up callbacks to Repo (actually invoke them)
The brakes the interface for the repo functions. Just match against `{:ok, value}` and let it raise a `MatchError` if someone rolled back. In the future we should add a raising `transaction!` function and use instead.