Add null_responses arg that doesn't do anything.
Added test and fixed bug for _get_frequencies to support the case when
there are excluded column values.
Finishes implementation of skipping null responses and tests that this
works as expected.
Merge branch 'master' into display

Conflicts:
	outlierdetect.py
Fix bad merge in last commit.
Fix typo in comments.
Merge branch 'master' into display