Added drawable objects in scrimage.canvas

I think the Canvas code has a lot of boiler plate and isn't so efficient:
 to draw 10 lines on the canvas you create 10 times a Graphic2D and close it 10 times.

I introduced a trait Drawable and added a draw method to the Canvas that can draw a collection of Drawable at once.

You may refer to the test class to see how the code is used.
If you like the idea, I'll add more Drawable objects, and rewrite the Canvas 'drawXXX' methods using Drawable.

Note: As the current version of scrimage is not compiling, some bugs may have escaped my attention.