Fixed tests and added new test data for searching over multiple variant sets.

-Fixed bug in testAcrossVariantSets that prevented any common reference names
 from being found.
-Used pep8 tool to ensure style compliance for test_backend.py.
-Added example_4 variant set, which is a copy of example_3 but with the CHROM
 column edited to include reference names from example_1, example_2, and
 example_3.