tls: add minDHSize option to tls.connect()

Add a new option to specifiy a minimum size of an ephemeral DH
parameter to accept a tls connection. Default is 1024 bit.

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>