Fix #199 - make *nrepl-src* closable with 'q'

`nrepl-popup-buffer-mode` was misbehaving originally, but the recent
refactoring of the popup window quit/restore logic fixed the problem
and now it's ok to use `nrepl-popup-buffer-mode` together with
`*nrepl-src*`.
Merge pull request #230 from bbatsov/fix-199

Fix #199 - make *nrepl-src* closable with 'q'