Add support for server password

Server password is configured through the ircServerPass
variable. As indicated in RFC 1459, the server password
is sent before the nick & user parameters.
(https://tools.ietf.org/html/rfc1459#section-4.1.1)

This is especially useful when the WebIRC protocol is used on the IRC server. In most cases, WebIRC has to be used when the server offers an IRC web client.