Added draft implementation for external callbacks
Callbacks: Added validation callbacks
Callbacks: No multiple callbacks for now
Callbacks: Added some more basic acceptance tests
Callbacks: Added unit specs for Callback module
Model: Added specs for running validate callbacks
Collection: Consistent naming
Collection: Running callbacks for save, replace, create, delete
Fixed test_unit collection generator
Added generators for callbacks
Callbacks: Register callbacks in the model
Railtie: Add app/callbacks to the autoload_path
Specs: Made models/collections accessible from all acceptance specs
Added Timecop for acceptance specs
Refactored time stamping of models into callbacks
Model: extend ActiveModel::Naming

It seems that worked either way. I have no idea why that is, but the
documentation says this module should be extended.
Documentation: Added docs for callbacks
README: Added section for callbacks
Docs: Never ever hash passwords!
Collection: Renamed `replace` to `update`