Can't specify redis password

[kbyanc]

Classifier.init accepts a host, port, and db parameter, but doesn't support specifying a connection password.

If would be nice if Classifier.init could take an arbitrary redis connection instance so that callers can instantiate their own connection however they like and then pass that connection to bayesredis to use. Alternately, just accepting a URL that bayesredis passes through to redis's from_url() factory would achieve the same goal.