added a notify event and extracted growl to its own plugin

Hi.

This Pull Request is a follow up of https://github.com/bevry/docpad/issues/332, to move growl capabilities out of the core.

- It adds a new 'notify' event to docpad.events array in order to plugins to listen to it.
- docpad.notify() does not call growl directly anymore but simply triggers the aforementionned notify event
- the growl handling and instantiaton have been moved to its own plugin : docpad-plugin-growl https://github.com/Delapouite/docpad-plugin-growl