Updated behavior of searchVariants re: callset IDs, to match ga4gh/schemas#166

Added command line option to specify no call sets, updated tests

Modified client options that indicate which call set(s) to display.

Removed the -C/--calls option, and changed the behavior of the -c/
--callSetIds option so that omitting it indicates no call sets and
passing in '*' indicates all (passing in a comma separated list is
the same as before).

Ran cli.py through pep8 and made style edits.
Changed searchVariants in the wormtable backend to allow searching
over multiple variant sets

Updated comments for searchVariants

Made minor stylistic changes to searchVariants

Modified verifySearchVariants to take list of variantSetIds.

Changed the function in test_backends.py and modified all calls to the method
in that file to to pass in a list instead of a single ID.

Added function to test searching over multiple variant sets

Updated tests for searching across variant sets.

Added function to get reference names that are common to a list of
variant set ids, and used this to iterate over common reference names
in the tests. Also fixed the tests to test all permutations of all subsets
of variant sets, instead of all permutations of all variant sets.

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.

Ran server.py through pep8 and made stlye edits.
Merge pull request #33 from sidrahussain/master

Support searching over multiple variant sets
Merge pull request #34 from sidrahussain/svr_semantics

Fix GASearchVariantsRequest semantics