Initial commit of settings module

This module adds the concept of developer-defined 'settings' for each site which can be edited through the Wagtail admin. They are defined as normal models, which are registered with the settings system - much like snippets.

TODO:

* [ ] Allow easy access to settings from templates.
* [x] Add an icon for settings in the admin. It currently has none.
* [ ] Write documentation on how to add and use settings
* [ ] Work out what should happen around automatically creating missing
  settings objects

This branch is based off a discussion in #368