Circumvent problems (see #46) with SASS (>3.4) by allowing keyframes to be prefixed

Default (optional) prefix for keyframes is "skrollr-".

Something like this in your CSS/SCSS would work:

```
@-skrollr-keyframes testprefixed {
	skrollr-0 {
		left:0px;
	}
	skrollr-100 {
		left:100px;
	}
}
```

SASS > 3.4 only seems to complain about keyframes that start with a digit.