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.