Implement all attributes of Rect.

Passes all tests in pygame/test/rect_test.py.
run.sh script that builds pygame_sdl2, and then runs a python script.
Implement Surface.set_colorkey, .get_colorkey.
Add pygame_sdl2.import_as_pygame().

This is a function that modifies sys.modules to replace the pygame
modules with pygame_sdl2, making it easier to port pygame code to
pygame_sdl2.
Implement Surface.set_alpha, .get_alpha.
Implement Surface locking.
Implement Surface.get_at, .set_at, .get_at_mapped.
Implement Surface.map_rgb, Surface.unmap_rgb.
Implement Surface.set_clip, .get_clip.
Make testing more possible:

* Temporarly import some modules from pygame.
* Stub pygame.quit.
* Allow color to take 3-tuples.
Extract keycodes from SDL header.
Implement most of Color, except for conversions.
Support short hex color strings.