Need to be able to specify a trust root

[superchris]

We're running our app behind a reverse proxy, which means that the openid.realm parameter that gets generated is incompatible with where we end up redirecting back to. If we could specific a trust_root parameter in the omniauth.openid configuration we could get around this.

[mbleigh]

I'd be happy to accept a pull request if you can write in your own solution here. Thanks!