Add nginx configure hook

This adds a pluginhook for nginx-configure to be called after the main nginx configuration is called and before nginx is reloaded. This will allow other plugins to modify the nginx configuration
Merge pull request #189 from jeffutter/patch-1

Add nginx configure hook
Set X-Forwarded-Proto Header

Heroku passes the X-Forwarded-Proto header to indicate the protocol of
the incoming request[1]. Ruby's rack relies on this header to
determine if a request is HTTPS[2].

Also, the X-Scheme header was removed to eliminate redundant passing of
the scheme.

[1]: https://devcenter.heroku.com/articles/http-routing#heroku-headers
[2]: https://github.com/rack/rack/blob/master/lib/rack/request.rb#L77
Add X-Forwarded-Port and X-Request-Start

Adding additional proxy headers to match what Heroku provides. Also,
X-Request-Start is compatible with NewRelic's latest agent[1].

[1]: http://blog.newrelic.com/2013/02/21/using-new-relic-on-heroku-read-how-our-new-ruby-agent-measures-queue-time/
Merge pull request #204 from statianzo/forwarded-proto

Set X-Forwarded-Proto Header