Support http_realip module

This would support the `set_real_ip_from` and `real_ip_header` directives provided by the http_realip nginx module. *NOTE*: before this is merged, the currently-closed #28 should be re-opened and possibly expanded upon (i.e. to force nginx install before config file not only via `package.sls`, but also via `source.sls`). The reason is that this pull request causes the nginx.conf template to check 'nginx -V' on the minion, so the nginx binary must be installed before the nginx.conf template compiles.

If there is not interest to complete #28 then:

* this may be closed, or
* I can remove the `nginx -V` check and users must simply use realip via pillar if and only if they know their nginx was compiled with realip. However if a user unwittingly sets realip info in pillar but their nginx does not support it, nginx will fail to start.