Update auth request helper to accept callback.
Implement authenticate at startup.

Such that if a username and password are supplied authentication is
attempted in the ready middleware, this enables commands such as
session/get to return a session document for the user that specified the
request, ie:

rlx session set -s http://localhost:5984 -u mock-admin -p secret

Has a correct userCtx object.

Authentication information is stashed in req.login.auth so that the lazy
authentication for command invocation will not try another login if
authentication information already exists.
0.1.38
Update man pages.
0.1.39