Allow https with self-signed certificates

Hi, and thanks for publishing your miner.

When minerd is used across the internet, the server's rpc userpass
travel unexcrypted, allowing anyone en route to copy them and
empty the server's wallet. Using https: would prevent this
but minerd's SSL connection fails on self-signed certificates.

This change adds a "-cert file" option (cert:"file" in minerd.conf)
to tell minerd the filename of the server's self-signed certificate.