proposed LICENSE for google_auth_proxy

@jehiah I figured you might be amenable to this license (the MIT license)
because that's what nsq uses.

If so, we should probably get the OK from everyone who has commits in this
repo before merging.

(another license I was thinking of was the apache 2.0 license, because that's
what https://github.com/abbot/go-http-auth uses)

fixes #3