Adding Linear Regression, and refactoring Ridge Regression.
Centralizing linear data generator and mllib regression tests to use it.
Fixing typos in Java tests, and addressing alignment issues.
Center & scale variables in Ridge, Lasso.
Also add a unit test that checks if ridge regression lowers
cross-validation error.
Merge pull request #863 from shivaram/etrain-ridge

Adding linear regression and refactoring Ridge regression to use SGD