Add `nickname` to info hash.

Also, expose a `nickname_key` setting, defaulting to 'user'.

I'm adding this because:

1. It's [in the spec](https://github.com/intridea/omniauth/wiki/Auth-Hash-Schema#schema-10-and-later).
2. At some point GitLab starting [using it to populate its username](https://github.com/gitlabhq/gitlabhq/blob/master/lib/gitlab/oauth/user.rb#L71).

I don't know for sure that this is the best way to handle that, as I feel as though GitLab is kind of Doing it Wrong™, but that's your call.

Incidentally, do you have any plans to make another release and push it to rubygems at any point? I'd like to use this in production but I like releases...