Added slidechange event & a few methods which may or may not be useful...

- A 'slidechange' event is now emitted when the slide changes.
- Added a toggleControls method with a public accessor on $wrapper.
- Added a play method with a public accessor on $wrapper.
- Added a stop method with a public accessor on $wrapper.

These were all added because I needed to hide the controls & display a play button on the first slide (with automatic set to false), and then show them again on subsequent slides. 

Thought this stuff may prove useful upstream if it's not too specific to my use case.