[SPARK-6841] [SPARKR] add support for mean, median, stdev etc.

Moving here from https://github.com/amplab-extras/SparkR-pkg/pull/241
sum() has been implemented. (https://github.com/amplab-extras/SparkR-pkg/pull/242)

Now Phase 1: mean, sd, var have been implemented, but some things still need to be improved with the suggestions in https://issues.apache.org/jira/browse/SPARK-6841

Author: qhuang <qian.huang@intel.com>

Closes #5446 from hqzizania/R and squashes the following commits:

f283572 [qhuang] add test unit for describe()
2e74d5a [qhuang] add describe() DataFrame API