Add 2 arguments to websockify.WSRequestHandler

This patch adds 2 arguments to websockify.WSRequestHandler for security:

* file_only: returns 404 response if non-file contents are requested.
             Required to disable directory listing.

* no_parent: returns 403 response if contents out of the web root are
             requested. Required to disable directory traversal.
Merge pull request #84 from nttdatainc-openstackers/master

Fix for issue #83