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
I rather we stick to one as well, I just used factory girl because I found some resources for it. It also keeps the code clean having the creation code somewhere else (the hash for creating an assignment is huge!). I can definitely look into using just the rspec tools if you think its just as good, I just want to make sure anything that we use here has enough resources for a new developer to start testing with ease.