Adding Linear Regression, and refactoring Ridge Regression.

Adding vanilla Linear Regression (no regularization) and refactoring Ridge Regression (Linear Regression + L2 Regularization) to use SGD -  MLlib. Adding test cases (scala+java) and augmenting the data generators for test support.

See https://github.com/mesos/spark/pull/765 for prior discussion of a similar pull request. Now refactored under @shivaram's updated GLM framework.