Adds X-Forward-For HTTP header support to Gunicorn

[hkeeler]

In order to get the "real" IP of an HTTP client, and not the upstream reverse proxy's IP, Gunicorn needs to support the X-Forward-For HTTP header.

For more details, see:

• http://gunicorn-docs.readthedocs.org/en/latest/settings.html#forwarded-allow-ips
• http://gunicorn-docs.readthedocs.org/en/latest/deploy.html

[hkeeler]

@awolfe76, could you check this one out? Just a one-line config change.

[awolfe76]

:+1: