1. prevent credential with more than two \u0000 seperaotr when using
PLAIN SASL mechanism
2. add test to cover #1
tweak test
using static factory method to create State object
refactor the State hierarchy
1. rename StreamElementExtractHandler to XmppXMLStreamHandler
2. rename XmppNamesapce.CONTENT to XmppNamespace.CLIENT_CONTENT
send close stream tag along the error message to client while an error
happen
Write javadocs for State related interfaces/classes