Add tooltips for transparency buttons

As suggested on Twitter: https://twitter.com/dblooman/status/482095919794761728

![screen shot 2014-06-26 at 1 59 35 pm](https://cloud.githubusercontent.com/assets/38924/3398058/620ab1f8-fd29-11e3-8f0c-3fa9cd6d8bd7.png)

cc @probablycorey