PetPlugin: moved all the commands to the separated classes.
Merge pull request #129 from rexim/pet-commands-refactoring

PetPlugin: moved all the commands to the separated classes.
Plugins: refactor plugin interface with traits. Remove TestPlugin.
README: remove the test command.
Core: prepare collections for room and user receivers.
Core: implement room notifications. Close #47.
PetPlugin: whitespace fix.
MucMessageHandler: control participant list through the special methods.
Core: add ParticipantProcessor and its infrastructure. Close #127.
JabberProtocol: reply with status when sending the messages. Close #135.
HealCommand: grammar fix.
Merge branch 'origin/master'
KillCommand: fix the grammar.