Use tls.DialWithDialer instead of tls.Client and remove irc.netsock

This change removes the need to store the separate netsock for TLS connections. Of more interest, the change facilitates the passing of the zero configuration for the TLS parameter when the IRC server you are connecting to has proper SSL configuration. This is explicitly disallowed when using tls.Client per http://golang.org/pkg/crypto/tls/#Client. 

tls.DialWithDialer requires requires Go >= 1.3, so if supporting older versions is a consideration this change should not be applied.