Add smtp gateway as user
User authentication

Authentication mechanism requires two parameter err and user - https://github.com/node-xmpp/node-xmpp-server/blob/master/lib/authentication/mechanism.js
When err is null, user must not be null for successful authentication.
Merge pull request #249 from srajbr/patch-1

User authentication
1.0.0-alpha2
Merge branch 'master' of github.com:node-xmpp/node-xmpp into issue/237

Conflicts:
	examples/c2s.js