Animation duration as option

[peirix]

We have a ticker that's supposed to tick every minute, and the animation looks really weird. So it'd be good to have duration as an option. I've modified the source, and it seems to work.

this.options = {
  delay: options.delay || 1000,
  separators: options.separators != null ? options.separators : false,
  autostart: options.autostart != null ? options.autostart : true,
  duration: options.duration || 250
};

And then passed in when calling flip

this.flip(parts.eq(1), digit, this.upper, this.options.duration, function() {});
this.flip(parts.eq(3).html(digit), digit, this.lower, this.options.duration, function() { ... });