First go at adding contour support. Still needs a lot of refactoring.
Refactored the code a little bit.

Added the contours getter. Still not working (gets one single contour
as the whole image.) Needs some more work.
Got the contours to work.

Apparently, I forgot to run the canny edge detector on the image
beforehand. D'oh!
Merge pull request #1 from andremedeiros/wip

Added Image#contours, Image#draw_contours and Image#canny