StatCounter on NumPy arrays [PYSPARK][SPARK-2012]

These changes allow StatCounters to work properly on NumPy arrays, to fix the issue reported here  (https://issues.apache.org/jira/browse/SPARK-2012).

If NumPy is installed, the NumPy functions ``maximum``, ``minimum``, and ``sqrt``, which work on arrays, are used to merge statistics. If not, we fall back on scalar operators, so it will work on arrays with NumPy, but will also work without NumPy.

New unit tests added, along with a check for NumPy in the tests.

Author: Jeremy Freeman <the.freeman.lab@gmail.com>

Closes #1725 from freeman-lab/numpy-max-statcounter and squashes the following commits:

fe973b1 [Jeremy Freeman] Avoid duplicate array import in tests
7f0e397 [Jeremy Freeman] Refactored check for numpy
8e764dd [Jeremy Freeman] Explicit numpy imports
875414c [Jeremy Freeman] Fixed indents
1c8a832 [Jeremy Freeman] Unit tests for StatCounter with NumPy arrays
176a127 [Jeremy Freeman] Use numpy arrays in StatCounter