[css3-animations] animation-direction values

The initial value of animation-direction is 'normal'. To be consistent with
'marquee-direction' (and also to be clearer in contrast with 'reverse'), I
suggest renaming it to 'forward'.

   http://dev.w3.org/csswg/css3-animations/#the-animation-direction-property-
   http://www.w3.org/TR/css3-marquee/#the-marquee-direction

~fantasai