Changes

Added Invites (#223)
* Added Invites
* Fixed permission checking
* Added checks and changed how invite expanding works
* Added Invite.Guild and Invite.Channel
* Added javadoc
* Made proposed changes
* Fixed NPE when guild icon is null
* Added JDAClient#acceptInvite()
* Removed old commented out invite code from JDA 2.0
* Added Invite#delete()
* Fixed a spelling mistake in javadoc and added Invite#getJDA()
* Added Channel#createInvite() and InviteAction
* Added missing javadoc
* Added copyright headers
* Removed old commented out imports
* Added Invite#accept() and JDAClient#acceptInvite(String) and made the
rest of the requested changes
* Fixed invite related docs
* Added InviteAction#setMaxAge(long, TimeUnit) overload,
 fixed an NPE when resetting maxAge and maxUses to default and
 added missing @throws
 (commit: 240adf3)