allow query databases to be passed into BlastxMapping.
allow query databases to be passed into BlastxMapping.

docs fix needed + test, too.

theoryno3/pygr,cjlee112/pygr,ctb/pygr,cjlee112/pygr,ctb/pygr,ctb/pygr,RitwikGupta/pygr,theoryno3/pygr,ctb/pygr,ctb/pygr,RitwikGupta/pygr,GenDataPro/pygr,GenDataPro/pygr,GenDataPro/pygr,cjlee112/pygr,RitwikGupta/pygr,theoryno3/pygr,cjlee112/pygr