text: Title: Validation Curve
  Body: Create validation curves to finish implementing the two types of validation curves in scikit-learn. 
  
  ### Proposal/Issue
  
  Scikit-learn provides two methods for diagnosing a model's sensitivity to bias vs variance as [validation curves](http://scikit-learn.org/stable/modules/learning_curve.html): 
  
  - [`learning_curve`](http://scikit-learn.org/stable/auto_examples/model_selection/plot_learning_curve.html)
  - [`validation_curve`](http://scikit-learn.org/stable/auto_examples/model_selection/plot_validation_curve.html#sphx-glr-auto-examples-model-selection-plot-validation-curve-py)
  
  We've already implemented the learning curve in #275 and is being moved to model selection in #377 -- to finish this up we should add the validation curve plot. 
  
  ### Code Snippet
  
  ![Validation curve](http://scikit-learn.org/stable/_images/sphx_glr_plot_validation_curve_001.png)
  