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
How about adding FactoryGirl's linter config? See https://github.com/thoughtbot/factory_girl/blob/master/GETTING_STARTED.md#linting-factories

We can pull all the FactoryGirl-related configs in a file at `spec/support/factory_girl.rb`, as was shown in the sample code in the link.