Add nrepl-clojure-version function

`nrepl-clojure-version` returns the Clojure version of the underlying
process as a string. This function will allows us to evaluate different
Clojure code depending on the version (recall the current problems with
the display of documentation in Clojure 1.5.0).
Merge pull request #299 from bbatsov/clojure-version

Add nrepl-clojure-version function