Export the CoffeeScript REPL.

This is done by adding a root level wrapper script for
lib/coffee-script/repl, similar to how the register script is wrapped.
This allows user programs to embed a CoffeeScript REPL without digging
into CoffeeScript's internals.
Merge pull request #3448 from leedm777/export-repl

Export the CoffeeScript REPL.