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
+1 on using stubs. One thing though: if we use FactoryGirl's `build_stubbed` for stubbing, then we probably shouldn't mix it with `rspec-rails`'s `stub_models`. We can still use `rspec-mocks`'s `allow(...).to receive(...).and_return(...)` on the stubbed object created by FactoryGirl to stub individual methods.