registerWithElements:error API

Fixes #232
XEP-0045: added methods to XMPPMessage category to handle room subjects, have XMPPRoom check incoming messages for room subjects and use them to set the roomSubject property
XEP-0045 Code Clean Up
Bug Fix: Message value on XMPPRoomMessageCoreDataStorageObject is not saving on CoreDate
Bug Fix: Message value on XMPPRoomMessageHybridCoreDataStorageObject is not saving on CoreDate
Merge pull request #239 from sergiosvieira/master

XMPPRoomMessage MessageStr Bugs