Detect vendor prefixes for CSS numeric value detection

[zpao]

If we add support for flex in #836, we need to support vendor prefixes as well.

Right now we just hard code the list of properties and check the object (https://github.com/facebook/react/blob/master/src/dom/CSSProperty.js#L24) to see if we should allow a numeric value unchanged. flex will need to support checking vendor prefixed values, so I think we should change our lookups.

We should still hard-code the values, but not include all of the possible prefixes. Instead we'll add a method which checks the object and falls back to stripping the prefix (I think /^-\w-/ should find the prefix) and then checking.
I think we should call a method

[sophiebits]

Vendor prefixes are like .style.webkitAnimation from the JS side.

[zpao]

Truth. Though as we just discovered, Webkit (or at least chrome) lets .style['-webkit-animation'] work. I don't think the same for -moz- works in Gecko

[sophiebits]

Fixed by 5abcce5.