Adds usage syntactic sugar for gulp-notify

Hi!

I've added some syntactic sugar for usage with `gulp-notify` and custom reporters, which, I think, works much better. I've updated your documentation to reflect the cleaner way to use it.

I've also bumped the gulp-notify dependancy version to the version having the syntactic sugar.

By the way; I think you should consider having `gulp-notify` as a dependancy and return the wrapped notifier from your plugin - simplifying the use of `gulp-notify-growl`.