#12 Button for Directing to Next Rejected Answer

* Add method to answer model for getting next rejected answer
* Add button to be used for redirecting to next rejected answer
Access Next Rejected Answer from the Controller

* Add scope to answer model for getting all rejected answers of a user
* Add an instance variable to the review controllers to store rejected
  answers of a user
* Allow the next_rejected answer to be accessed by the view if the user
  is a mentor or a recruiter and there are at least two rejected answers
* Add rspec test to the answer model for testing functionality for
  getting rejected answers
Merge pull request #46 from femnad/issue_12

 #12 Button for Directing to Next Rejected Answer