Relax keyframe selectors

The changes adds the ability to parse basic [Skrollr Stylesheets](https://github.com/Prinzhorn/skrollr-stylesheets).

It allows numerical keyframes-selectors and easing function syntax, eg:

    @-skrollr-keyframes one {
        top {
            left: 100%;
            opacity[swing]: 0.0;
        }
        100 {
            left: 10%;
            opacity: 0.5;
        }
        bottom {
            left: 0%;
            opacity: 1.0;
        }
    }

* numerical keyframes-selectors
* `top`, `bottom` keyframes-selectors
* easing function syntax