Fixed JS error when background-position has either "left" or "top" (fixe...

...s #14)

When `"left"` is used, `positions[ pos[0] ]` evaluates to `0`. Since this is `false`-y, the calculation moves onto `parseFloat( pos[0] ) / 100`, which evaluates to `NaN` (since it's trying to do `parseFloat("left")`). (A similar case occurs with `"top"`.) This leads to a JS error later on.