fix #194 - stacktrace buffer was not respecting nrepl-popup-stacktraces

In nrepl-default-err-handler we were checking only if the major mode is
nrepl-mode which caused the nrepl-popup-stacktraces setting to be
ignored in clojure-mode. As a bonus I've simplified a lot the code
that was used to check the buffer's major mode.
updated nrepl-popup-stacktraces docstring to reflect the fix for #194
Merge pull request #209 from bbatsov/fix-issue-194

Fix #194 - stacktrace buffer was not respecting nrepl-popup-stacktraces