Created InviteMessage class
ACE-93
Added inviteUser to ChatService and ConcreteChatService
Updated Participant to have a status of "INVITED"
ACE-93
Corrected invite message "to" path

ACE-93
Made message TYPE static
Exception throwing for non-existent users in ConcreteChatService

ACE-93
Updated javadoc on ChatService

ACE-93
Merge pull request #52 from ualerts-org/createInviteMessage_ACE-93

Create invite message ace 93