Reduce logging of reverse proxy IP lookup and use configured logger

Instead of logging the notice that django.axes looks for a HTTP header
set by a reverse proxy on each attempt, just log it one-time on first
module import.
Also use the configured logger (by default axes.watch_login) for the
message to be more consistent in logging.
Merge pull request #77 from eht16/reverse_proxy_log_reducing

Reduce logging of reverse proxy IP lookup and use configured logger. Fixes #76