Moved display element tests to their own file
Moved container tests to test_container_elements.py
Added ProgressBar display element
Removed unused elements.py
Added Box container element
Changed interface of container objects
Removed unused elements.py
Added Zebra container element...

like a Stack, but stripes the default formats of its contents.
Changed interface of container objects
Added unit tests for Box element...

and yet again found that I'd made a small mistake once it came to actually
doing some unit testing!
Added unit test for Zebra element, fixed niggles
Merge branch 'box'

Added a box container element which will draw a box around a given element.
Merge branch 'zebra'

Added a Zebra container element, which behaves like a Stack, but stripes the
formats for each row.

Conflicts:
	test/test_container_elements.py