conkeror go to buffer

In conkeror, you're trying continuously to think emacs, so many commands and shortcuts just make sense when you're browsing.

A nice way to move through buffers is obviously c-x b, but from time to time I was typing M-g M-g as if it was a 'goto-line' in emacs, but meaning goto-buffer. 

So let's go and use js to have this goto-buffer