[SPARK-6164] [ML] CrossValidatorModel should keep stats from fitting

Added stats from cross validation as a val in the cross validation model to save them for user access.

Author: leahmcguire <lmcguire@salesforce.com>

Closes #5915 from leahmcguire/saveCVmetrics and squashes the following commits:

49b507b [leahmcguire] fixed tyle error
67537b1 [leahmcguire] rebased
85907f0 [leahmcguire] fixed name
59987cc [leahmcguire] changed param name and test according to comments
36e71e3 [leahmcguire] rebasing
4b8223e [leahmcguire] fixed name
4ddffc6 [leahmcguire] changed param name and test according to comments
3a995da [leahmcguire] Added stats from cross validation as a val in the cross validation model to save them for user access