Style buttons to match new design

Fixes [Task 357](https://assemblymade.com/helpful/wips/357).

**What it looks like in-browser**
![screen shot 2014-04-21 at 10 06 09 am](https://cloud.githubusercontent.com/assets/65468/2754850/3f42f44a-c95e-11e3-8d2e-59aa862891f3.png)

**What it looks like in helpful-forms-04-15-14.psd**
![screen shot 2014-04-21 at 10 09 15 am](https://cloud.githubusercontent.com/assets/65468/2754860/8bb7d7fa-c95e-11e3-86c6-fbade9d8c1e9.png)


Some of the code is a bit hackey; couldn’t get SASS arithmetic going on some variables for some reason. Suggestions are welcome!