Improve design of Camera and Image.

* Add defaults to camera constructor.
* Make Camera.set_dimensions public.
* Add Image.get_width and Image.get_height.
Move Pointer int moustrap.gui.
Moving test_*.py to tests/.

Having tests in the same directory is cluttering up my mind.
Give me a good reason that I haven't thought about and I'll
consider putting them back.
Add bin/mt-clean.
Add moustrap.gui.Gui to collect public mousetrap.gui functions.
Encapsulate screen access into moustrap.gui.Gui.
Move Image and related functions to mousetrap.image.
Add bin/mt-kill-runaway to kill moustrap if it gets away from me.
A little doc for main.py
Add mousetrap.pointers package to hold pointer modules.
Each pointer module must define a Pointer class that inherits
mousetrap.pointers.interface.Pointer.