basic support for after_fork callbacks.

this is a very basic implementation for #16. I currently only implemented global `after_fork` callbacks. The functionality is completely separated from the commands. Some thoughts:

* We could put `invoke_after_fork_callbacks` on the `Command` class
* We could have a simple method on `Command` to first invoke the callbacks and then call it.

```ruby
class Command
  def run(args)
    invoke_after_fork_callbacks
    call(args)
  end
end
```
This would give the commands more flexibility over the execution process.

The current implementation is very straight forward and should serve as a point to start a discussion.