Remove local copies of websockify

This commit removes local copies of websockify.
Instead `utils/launch.sh` performs the following logic:

If `utils/websockify` exists, use `utils/websockify/run` (if the latter
does not exist, or is not executable, fail, since this is probably a
mistake).

Otherwise, check to see if websockify is installed somewhere (try
`which websockify`).  If it is, use that.  Otherwise, clone
websockify from github, and tell git to ignore that directory.

Packaged versions of noVNC should simply list websockify as a
requirement.  The debian packaging has been updated to reflect
this.

Closes #433