The HTML5 dataset API does not allow uppercase letters

Hi Bartek Szopka!

I love impress.js, it is truely a tidy, compact and powerful library!

When trying to allow the slides to scale to the full window size on large monitors I encountered a problem setting the "maxScale" value through the HTML5 dataset API, I traced it down to uppercase letters when initializing "maxScale".

Cheers,
David