Minor tweaks, improvements, spellchecking, standardization

Minor spelling fixes in description; adds -ms- prefix to @mixin scale
for free IE9 support; adds variables to @mixin shadow; adds default duration
and standardizes capitalization of @mixins scaledown, scale up, fadein;
moves functions black and white to directly under opacity; switches
names of center-h and center-h—unk for accuracy
This implementation still allows users to simply call @include shadow(blue) if that's all they want. I figure the way to go with this kind of thing is "non-mandatory functionality"