fixed nrepl-symbol-at-point's behaviour when at the beginning of the line
Added contribution guidelines.

The contents of CONTRIBUTING.md will taken into account by GitHub
and a link to them will be displayed to users creating new issues.
Removed the mention of new modules from CONTRIBUTING.md
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.
Implement nrepl-mode as a derived mode.

These days it's idiomatic to implement major modes
in terms of the define-derived-mode macro. It simplifies
the definition by using convention over configuration and
gives you the ability to reuse code from parent modes.
This particular change should transparent for our users.
updated nrepl-popup-stacktraces docstring to reflect the fix for #194
Merge pull request #210 from bbatsov/derived-mode

Implement nrepl-mode as a derived mode.
Merge pull request #209 from bbatsov/fix-issue-194

Fix #194 - stacktrace buffer was not respecting nrepl-popup-stacktraces
Merge pull request #207 from bbatsov/contributing

Added contribution guidelines
Merge pull request #189 from bbatsov/fix-nrepl-symbol-at-point

Fixed nrepl-symbol-at-point's behaviour when at the beginning of the line