text: Title: Finish up CVScores visualizer
  Body: CVScores is a new visualizer under yellowbrick/model_selection. Here are a couple things left to do and small enhancements:
  
   - [x] Add documentation ([done](http://www.scikit-yb.org/en/develop/api/model_selection/cv.html))
   - [x] Add test cases
   - [x] Add a legend that labels the average cvscore dotted line with the numeric value of `self.cv_scores_mean_`
   - [x] Adjust the ylim so that `CVScores` plots will be more easily comparable across different models.
  