First draft of testing guide using RSpec.

This is a very rough draft. I'm still exploring what exactly our testing guide needs, any feedback will be appreciated.  @david-yz-liu @zhangsu @arkon
By the way, I think it's possible to just link to sections like `#controller-specifications`. It seems to work fine in the README [here](https://github.com/cssu/cssu.cdf.toronto.edu).