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.
Merge pull request #210 from bbatsov/derived-mode

Implement nrepl-mode as a derived mode.