Add BalanceText 1.2.0

Source: https://github.com/adobe-webplatform/balance-text
License: Apache License, Version 2.0

This is a polyfill for a [proposed](http://adobe-webplatform.github.io/balance-text/proposal/index.html) addition to CSS. See [this blog post](http://blogs.adobe.com/webplatform/2013/01/30/balancing-text-for-better-readability/) for more information.