Refactor ConcreteChatService

ACE-86
Remove unnecessary Iterator import

ACE-86
Fix invocations of findDefaultConversation

ACE-86
Remove commented-out code in ChatService.

ACE-86
Remove addService from the ChatService API

ACE-86
Modify javadoc on ChatService.getConversation() and .joinConversation()
Make ConcreteChatService.addConversation() private

ACE-86
Make only ConcreteChatService.joinConversation() make new conversations

ACE-86
Rename ConcreteChatService.addConversation to .createConversation and
modify the javadoc

ACE-86
Merge pull request #45 from ualerts-org/RefactorChatService_ACE-86

Refactor chat service ace 86