Added support for alternative ssh ports

I had a need to use a port number other than 22 for my deployment, I have edited this to support the use of STAGING_SSH_PORT, PROD_SSH_PORT etc in WP-config.php
Moved proposed ssh_port setting to _prepare_and_extract

Avoiding the repetitive handling of this.
used intval on proposed ssh_port instead of escapeshellarg
Merge pull request #6 from nubz/master

Added support for alternative ssh ports