Relax keyframe selectors

[jo]

The changes adds the ability to parse basic 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

[tj]

I don't really want to make up syntax at the css-parse level

[jo]

I was kind of doubtfully, too, but I think its just about relaxing the parser a bit.

[tj]

yeah could be worse haha, a few tests would be good

[jo]

ok I squashed the commits and added some tests

[tj]

awesome! thanks