Forward secrecy support (Ephemeral Diffie-Hellman)

In order to support the EDH ciphersuites, a server requires a file
containing Diffie-Hellman parameters to be specified. This patch adds
support for such a filename to be passed in to EventMachine's start_tls.

Docs:

* http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_dh_callback.html
* http://www.openssl.org/docs/ssl/SSL_CTX_set_options.html
* https://wiki.mozilla.org/Security/Server_Side_TLS
Use server's priority order for the cipher list

By default OpenSSL uses the order of ciphers specified by the client,
and ignores the order in the cipher list you configure on the server.
That causes the guidance for cipherlist strings (e.g.
https://wiki.mozilla.org/Security/Server_Side_TLS) to break down.
Merge pull request #2 from rapportive-oss/review.forward-secrecy

Forward secrecy support (Ephemeral Diffie-Hellman)