fix logistic loss bug
Added GradientDescentSuite
add the comments about SPARK_WORKER_DIR

this env variable seems to be forgotten …
Merge pull request #349 from CodingCat/support-worker_dir

add the comments about SPARK_WORKER_DIR

this env variable seems to be forgotten

in many cases we need to set this variable, e.g. in EC2, we have to move the large application log files from the EBS to the ephemeral storage
Merge pull request #337 from yinxusen/mllib-16-bugfix

Mllib 16 bugfix

Bug fix: https://spark-project.atlassian.net/browse/MLLIB-16

Hi, I fixed the bug and added a test suite for `GradientDescent`. There are 2 checks in the test case. First, the final loss must be lower than the initial one. Second, the trend of loss sequence should be decreasing, i.e., at least 80% iterations have lower losses than their prior iterations.

Thanks!