Responsive custom reCaptcha widget

Fixes #779.

![screen shot 2014-10-24 at 4 53 54 pm](https://cloud.githubusercontent.com/assets/3108007/4777962/0fcae308-5bda-11e4-8068-7eb599cd79d5.png)

![screen shot 2014-10-24 at 5 03 16 pm](https://cloud.githubusercontent.com/assets/3108007/4777992/683d69ca-5bda-11e4-877e-be54904c9e66.png)