[Azoth Abbrev] Added abbreviation plugin stubs.
[Azoth Abbrev] Added Abbreviation.
[Azoth Abbrev] Added AbbrevsManager.
[Azoth Abbrev] Register metatypes.
[Azoth] Added CommandException for IProvideCommands.
[Azoth Abbrev] More sanity checks.
[Azoth Abbrev] Added /abbrev and /listabbrevs commands.
[Azoth Abbrev] Added /unabbrev.
[Azoth Abbrev] AbbrevsManager::Process().
[Azoth Abbrev] Store abbrevs in length-decreasing order.
[Azoth] Apply commands before hookMessageWillCreated().
[Azoth Abbrev] Hook on message creation & really process msgs.

Closes #1653.
[Azoth Abbrev] Slightly clarified docs.