Fix #672 (?)

Add Later to supybot.commands.defaultPlugins.

Some concerns:
* This makes Later to be always loaded, doesn't this?
* Is Later really enough important to be always loaded?
    * Most of people are usually loading it.
* If Later is important plugin, aren't some others than these too?
    * Ctcp, Google, Web, NickCapture, Seen, Time, Topic?
* Will this make Limnoria bloated?

**Comments wanted!**