Refactored Message to have a subType property
Made getType and getSubType abstract in Message
Implemented getType and getSubType in ChatTextMessage
Made RosterMessage abstract and implemented getType to return 'ROSTER'
Created RosterAddedMessage, extending RosterMessage,and implementing
getSubType to return 'ADDED', also added a userName property
Created RosterRemovedMessage, extending RosterMessage, and implementing
getSubType to return 'REMOVED'
Created RosterContentMessage, extending RosterMessage, and implementing
getSubType to return 'CONTENT'
Refactored PresenceMessage due to Message refactoring
Refactored ConcreteConversation to implement new roster message types
- added implementation for getType and getSubtype
Made 'chat' a constant in ChatTextMessage.
Added javadocs to Message and RosterMessage
Set the getType method in RosterMessage as final.
Set the getRosterAddedMessage, getRosterRemovedMessage, and
getRosterContentMessage as protected.
Merge pull request #20 from ualerts-org/createRosterRemovedMessage_ACE-50

Refactored Message to have a subType property