Proxy off to PHP better.

The preserve_host option on rack-rewrite seems a bit off. I would assume it
would mean it keeps the hostname requested. But instead it replaces it with
the internal hostname (the host we proxy to). There is a ticket related to
this (https://github.com/jaswope/rack-reverse-proxy/issues/11).

In addition, even if it pulled from the right host it also doesn't include
the port. There is also already a ticket for this
(https://github.com/jaswope/rack-reverse-proxy/pull/17).

To resolve this I simply avoiding using the preserve host option. This allows
the original host to pass on through to PHP (it doesn't seem to need to be
explicitly set, PHP already know it).

Both of these option (wrong host and missing port) cause some PHP scripts,
like Wordpress, who are trying to redirect to a canonical URL to get caught
in an infinate loop.

I am also now stripping out the index.php from the PATH_INFO before proxying
off. The PHP webserver has it's own built-in method of looking for the
index.php. So we add it for detecting the PHP handler should be called but
then remove it to let PHP do things it's own way. This helps for script like
Wordpress that try to direct to a canoncial URL. It prevent it from bouncing
between index.php and the root directory. It may be slightly incorrect for
cases where the user explicitly chose to pull up the index.php script. But
in the real world I don't know where that would cause a problem and making
this change solves a real world problem for Wordpress.

Finally, I am passing on the HTTP_X_FORWARDED_FOR header. Although not
standardized it is fairly widely and provide a way for the script to determine
the IP address of the real browser since REMOTE_ADDR will be set to the
proxy server.