Add Area class.
Add tests for Area.
Deal with zero heights and widths better.

Conflicts:
	lib/vedeu/output/area.rb
Started working on Viewport concept for scrolling interfaces.
Add Content class to track a virtual cursor within the content of the interface.
Add #refresh method to update the interface after the cursor moves.
Use correct range operator to remove requirement to alter value of #viewport_height.
Experiment with options.
Add Viewport class.
Add unused defaults.
Experimental stuff for scrolling, cursors and geometry.
Remove unused code.
Simplify by removing empty check and using #key?

- Whitespace.
Add more documentation, also;
- Fix whitespace.
- Remove commented out code.
Move exceptions into correct file.
Make #y and #x public.
Add Offset class;

Calculates offset for a cursor based on an Area and a Cursor.
Remove unused exceptions and methods, also;

- Extract methods and reduce complexity.
Simplify Area by using attributes rather than arguments.
Render the cursor if the interface is focussed.