Follow symbolic links in launch.sh

Previously, in launch.sh, `$HERE` was the directory of `$0`.
However, if `$0` was actually a symlink, `$HERE` would be
wherever the symlink was, which could cause issues (for
example, the script wouldn't be able to local `$WEB` or
`$WEBSOCKIFY` properly).

Now, `$HERE` looks at whatever `$0` points at instead.

Closes #447.