Refactored SavedDOI into Cart model

* aded tests for Cart model
* added methods for deleting or adding DOIs in bulk
* make sure we never access session[:dois] directly
* refactored update_session
* added tests
* renamed articleLimitReached? to article_limit_reached?

This closes #36 and #37. Need to fix the controller test for `update_session` which is currently timing out.