Adding a Common Lisp wrapper.

Features:
  + configurable via environment variables
  + can skip the actual launching of the lisp implementation (source it
  with NIX_LISP_SKIP_CODE=1 to get all the settings)
  + currently supports SBCL, CLisp, ECL
  + determines lisp implementation from NIX_LISP_COMMAND variable or
  from buildInputs
  + sets ASDF search path for packages using buildInputs