Duration Option

I've added an option where you require a fixed duration for the animation. Let's say 1 second. So it calculates the speed of the animation based on the number of animated characters to make sure each transition takes the same length of time.

There is also a bug fix where HTML was finding its way into it. It now uses .text instead of .html where relevant.

I'm about to fork this code quite dramatically - so I thought I'd commit this before I make it completely unusable!