Add "beta stage" header

Per #6. @meiqimichelle @gbinal 

Set it to "beta" instead of "alpha". I can be persuaded to change it, but it seems more "beta" to me based on [the Dashboard stage definition](https://18f.gsa.gov/dashboard/stages/): **Stage and test a live, robust demo on the public web for use by a subset of the target audience.**

Please merge if it looks good to you. As currently seen on the internal Hub:

![beta-stage-header](https://cloud.githubusercontent.com/assets/301547/5515785/b4438850-8848-11e4-97e5-f912a870a672.jpg)