crypto: expose ECDH class

For consistency with the rest of the crypto classes, exposes the ECDH
class. Originally, only the createECDH function was exposed, and there
was no real reason to hide the class.

PR-URL: https://github.com/nodejs/node/pull/8188


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>