Handle SSH URIs with connection parameters

SSH URIs as described on the RFC draft http://tools.ietf.org/html/draft-ietf-secsh-scp-sftp-ssh-uri-04 admit a set of c-param components that can be used to pass additional information about the connection (i.e., the key to be used).

This patch extends URI to support it.

URIs of class URI::ssh now have a c_params method accepting/returning an array with the c-param components.