tls: add TLSSocket.getEphemeralKeyInfo()

Returns an object representing a type, name and size of an ephemeral
key exchange in a client connection. Currently only DHE and ECHE are
supported.

This api only works on on a client connection. When it is called on a
server connection, null is returned. When its key exchange is not
ephemeral, an empty object is returned.

PR-URL: https://github.com/nodejs/node/pull/1831


Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>