changes to get octoprint accessible behind a reverse-proxy

I've to add some changes to get octoprint accessible behind a reverse-proxy, in my case lighttpd.

The difference is, that not every proxy seems to send the "X-Script-Name" with the HTTP-request.

With this patch you can enable support for proxys who supply the sub-path. The header-field can be configured in OctoPrints config.

More info can be found here:
http://librelist.com/browser/flask/2010/7/21/running-behind-proxy-with-url-prefix-other-than/#350beec683d8fc02f50613e235e6a413