upgrade netty to 4.0.0.CR1
parse incoming presense subscription
Use visitor pattern to deal with different first level tag.
use visitor pattern to deal with SASL
deal with initial presence broadcast
Implement an XmppContextManager to manage all active XmppContext
add more log
inject XmppContextManager
deliver presence subscription request to contact
after deliver subscription request to contact, send a roster push to
user
ignore income <iq type="result"/>
fix a bug that always use same XmppContext to send roster push after
server deliver subscription request
rename 
FirstLevelElementVisitor => DefaultFirstLevelElementVisitor
Visitor => FirstLevelElementVisitor
Visitee => FirstLevelElementVisitee
rename
FirstLevelElementVisitor => ElementVisitor
DefaultFirstLevelElementVisitor => DefaultElementVisitor
FirstLevelElementVisitee => ElementVisitee