Added opacity option

You might want to have a review of my code, I'm sure it can be optimised. I only tested in Chrome 27.0.1453.110 so it might be worth doing some cross browser.

- Added opacity value to the section tag (fullscreen-opacity(0.x))
- Added a bool to the event listeners; if you reloaded the presentation
halfway through it would call both and then you'd have a leftover image
(with opacity, anway).
- Opacity mode creates a new dom element, and then removes it on
transition. Non opacity mode remains the same as it was, as it's
faster. Stops the DOM from repainting so much.
- Assigned the imageContainer/document.body into a var to stop multiple
requests