Switching to RSpec for testing

This is the first installment of the potential switch, it illustrates the format of model testing. If anyone has any input on how I can make these tests even better, while maintaining readability, I would really appreciate the help. I purposely only tested one method so that we can focus more on the style versus what is actually being tested. I'm hoping to decide on a style before this PR is merged.  @ferrous26 @zhangsu
Okay, I think you've convinced me. RSpec is all about readability, I guess hiding the creation code in another file defeats that purpose. 
I was trying to make it explicit that group instantiated at the beginning was different from the g instantiated in the blocks, but I guess it would be obvious even if I called it group.