add from_url() to connection pools, from_url() can parse UNIX domain socket URLs

i needed to construct a connection pool from a URL, so i added from_url() classmethods to both pool implementations (requested in #356).  i also added UNIX domain socket URL support while i was in here.  added test cases for ConnectionPool and BlockingConnectionPool.  i didn't see any tests for StrictRedis.from_url(), so i added two cases for that, too.