Add CSS3 transition (optional)

Hi there,

As I wanted to add some transitions for the opening & closing events, I decided to edit the plugin itself. I'm far from being a jQuery expert, so please review my code before merging the pull request.

In my case, I used https://github.com/daneden/animate.css to animate my modals:
 ```javascript
		$('.ta-modal').easyModal({
			top: 200,
			overlay: 0.2,
			transitionIn: 'animated bounceIn',
			transitionOut: 'animated bounceOut'
		});

Lemme know what you think!