Added support for relative positioned slides

Added support for relative positioning via the data-dx, data-dy and data-.dz attributes.

Many presentation layouts will be much easier to alter and update if the steps is relative positioned to each other.

I also added a possibility to write javascript expressions in the positional attributes so that the positioning could be based on window.innerWidth for example.

To make It a little more user friendly I also added a two keywords  pageWidth and pageHeight which will be replaced with window.innerWidth / window.innerHeight

The demo presentation is updated to use the new relative positioning as well