added diffie-hellman parameter functions. allows using DHE-* cipher suites

I had legacy clients that needed DHE-RSA-AES128-SHA and found that there was no way to specify the DH parameters (dh2048.pem, for example) on the context, so DHE was not supported even if I specified "ALL" ciphers.

I attempted to copy the same style and naming as the rest of the project.

Please let me know if there is something you'd prefer done differently; I wasn't certain what the filename should be, for example.