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
To speed up controller spec, we can stub methods like `current_user` to bypass the authentication.

```ruby
allow(controller).to receive(:session_expired?) { false }    
allow(controller).to receive(:logged_in?) { true }           
allow(controller).to receive(:current_user) { build(:admin) }
```
So no admin object needs to be persisted.