gerrit: add Auth mode to use a specific cookie file, add more methods

This is stuff needed for my Github->Gerrit bot.

Change-Id: Ie072d7b66be2b219ba48245305a8187446238c9e
Reviewed-on: https://go-review.googlesource.com/19380
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed