[MLLIB] SPARK-2303: Poisson regression model for count data

This pull request includes the implementations of Poisson regression in mllib.regression for modeling count data. In detail, it includes:
 1. The gradient of the negative log-likelihood of Poisson regression model.
 2. The implementations of PoissonRegressionModel, the generalized linear algorithm classes which use L-BFGS and SGD for parameter estimation respectively, and the companion objects with ``train'' interfaces.
 3. The test suites
    * the gradient/loss computation
    * the regression method using LBFGS optimization on generated data set
    * the regression method using LBFGS optimization on real-world data set
    * the regression method using SGD optimization on generated data set
    * the regression method using SGD optimization on real-world data set
 4. a Poisson regression data generator in mllib/util for generating the test data.

JIRA: https://issues.apache.org/jira/browse/SPARK-2303