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*`.
added a basic check for an existing REPL buffer in nrepl-jack-in & nrepl
Use nrepl-symbol-at-point instead of eldoc-current-symbol for eldoc.
release v0.1.6
bump version
Merge pull request #162.  pprint mode for REPL.