Add feature: Toggle tracing/untracing of a var at point.

add function: `nrepl-toggle-trace` - bound to `C-c` `M-t`
to trace the var at point.

Preconditions to make it work.

* clojure.tools.trace must be on the class path of the repl for this
  feature to work. Add it to your lein project.clj, like so:
  {:dependencies [org.clojure/tools.trace "0.7.5"]}

* The namespace of the buffer must be loaded into the repl (with
  e.g. C-c C-k)

This is a reopening of PR: https://github.com/clojure-emacs/nrepl.el/pull/329

Since the new structure for simple commands hasn't been implemented in master, I would like the tracing feature in nrepl using the current architecture.