Addition of a styleguide, for logged in users only and not publicly linked.

Anyone designing or developing for wagtail will need a reference for the available UI that can be used in their developments.

It still has a couple of "TODO"s left, but as this is mainly only consumable by developers and isn't linked anywhere for regular users of wagtail, this shouldn't prevent it being merged with master.

TODOs I'm particularly in need of help with are adding examples of the Chooser systems (Page, Image, Doc, Snippet) to the "Forms" section.
I think it would be worth adding a hook anyway, contrib or no, although in the interests of merging this without further delay we can leave hooks till later.