Added trust_xheaders config option to delegate it to HTTPServer.

This is neccesary if the notebook app is proxied behind a reverse
proxy that handles SSL.
Merge pull request #3147 from meteogrid/rproxy_support

Notebook support for a reverse proxy which handles SSL

Added a "trust_xheaders" config option to NotebookApp to enable the passing of the xheaders flag to tornado.HTTPServer. This is necessary so tornado can generate the correct wss:// scheme