The implementations of Poisson regression in mllib/regression. It includes:
  * The gradient of the negative log-likelihood of Poisson Model.
  * The implementations of PoissonRegressionModel and the generalized linear algorithm class. Here we can use either LBFGS or GradientDescent as the optimizer. So we implement both as class PoissonRegressionWithSGD and class PoissonRegressionWithLBFGS respectively. Also added the companion object PoissonRegressionWithSGD and PoissonRegressionWithLBFGS as drivers.
  * Test suites for the gradient/loss computation and the modeling/regression results of the generated and the real-world data set using LBFGS and SGD respectively.