Fix bug when creating more than 2 unnamed terminals

Hey @Silex can you take a look...

Currently on my emacs setup, if I do `nav/term` more than twice, it hangs trying to create a new terminal... I modified slightly the code there (stop relying on multi-term list and got the terminal list count via `buffer-list`)

Wanted you to check it out before I merge it to master

Cheers.

Roman.-