X-Forwarded-For improvements and README formatting

This PR is primarily to fix https://github.com/tjanczuk/iisnode/issues/340, by not adding a duplicate IP to the X-Forwarded-For header if it already is the last one listed. Basically, if the header already exists, we look at the end of the existing value, and if the remote host IP we would add is the exact same as the last IP of the existing header, we don't append our value.

While I was at it, I also tweaked the formatting of the HOWTO section of README.md so it looks a lot nicer.