Streaming mllib [SPARK-2438][MLLIB]

This PR implements a streaming linear regression analysis, in which a linear regression model is trained online as new data arrive. The design is based on discussions with @tdas and @mengxr, in which we determined how to add this functionality in a general way, with minimal changes to existing libraries.

__Summary of additions:__

_StreamingLinearAlgorithm_
- An abstract class for fitting generalized linear models online to streaming data, including training on (and updating) a model, and making predictions.

_StreamingLinearRegressionWithSGD_
- Class and companion object for running streaming linear regression

_StreamingLinearRegressionTestSuite_
- Unit tests

_StreamingLinearRegression_
- Example use case: fitting a model online to data from one stream, and making predictions on other data

__Notes__
- If this looks good, I can use the StreamingLinearAlgorithm class to easily implement other analyses that follow the same logic (Ridge, Lasso, Logistic, SVM).
We shouldn't assume the number of features is 2. We can default to `null` and throw an error if this is not set when `trainOn` is called.