Support for Snarl

Adding support for Snarl.

Because we always need to execute 2 commands (one for registering, one for showing the notification) I had to combine those into a single node-script that is executed. Otherwise it wouldn't work as Grunt is exiting the process and does not wait for the callback of the spawn for the first command.

I also added options that can be passed to snarl, like `timeout`. unfortunately i wasn't quite sure how to pass them through from the gruntfile to the notifier, so i left that up to you... meaning, there are no options passed yet, except for `image`, `title` and `message`.