Events, groups and focus.

- Add #unevent method to Events; this allows the deletion of an event by name,
    effectively deleting all associated actions.
- Add #reset to Events to remove all registered events. (Used for testing
    really.)
- Add #registered method to return all the names of the registered events.
- Fix bug in event triggering.
- Add Focus class to store interfaces by name and the current interface in
    focus. (Not fully integrated yet.)
- Move contents of output/ to support/.
- Extract #colour and #style attributes into Presentation class.
- Prefer #render over #output for client applications;
    Rather than defining a method #output in your view class, use #render
    instead, this feels more 'frameworky'.
- Add test for Vedeu.views
- Add tests for Group class.
- Add more documentation.
- Add alias_method `composition` to API which redirects to `views`.
- Move Store into Buffers;
- Add Groups class which is an index of interfaces by group name.
- Bump guard-minitest version to 2.3.2.
- Handle colour support better.
- Write warnings to log file if terminal is too small.
- Remove pointless logger tests.