Support3d transform

I was just implementing this with some pretty large image layers. And using top/left to move elements was pretty jolty.

I've added support for using translate3d rather than top/left where it is supported. Default to true. * This does however require a bump in jQuery dependency support to 1.8.0+


I've also implemented an option, allowing for use of this, on plax layers contained within fixed elements. Currently if the plax layer is a fixed element, and following scroll it would be calculated as outside the viewport, movement is stopped even though it is still visible. I've added an option to declare that the plax is fixed - and continue it's movement.



Updated all examples to use jQuery 1.8.0, bower.json, and README to document the 2 additional options above.