Raise events for grow and shrink actions

This change means that when autogrow grows/shrinks a textarea, it will then dispatch events to indicate this (Event won't fire until animation is complete, if the user has animation enabled). I've added some docs to the readme, but the code is pretty straightforward. 

It means the user's app can keep track of what's going on and update as appropriate.

As an example, my app has a textarea at the bottom of the screen, and when the box grows, I want to auto-scroll the browser down so that the full box is still visible.

What do you think?

It would also solve this issue: https://github.com/ultimatedelman/autogrow/issues/8

(Haven't updated the .min or json files btw)

PS - Thanks for making this plugin, it's been so useful!