Add rect method

[watzon]

Closes #7

It's tested and working, but I'm open to suggestions. It's based directly off of ChunkyPNG::Canvas::Drawing#rect.

[l3kn]

Nice!

I think it's reasonable to assume that x0 < x1 and y0 < y1
Clipping the rectangle against the canvas first, then using set instead of safe_set would probably make this a little bit faster.

I have not used crystal in a while and looking at my old code, the API isn't designed that well.

The canvas.circle takes a boolean filled argument,
canvas.fill_polygon has "fill" in its name and there's no way to draw just the outline of a polygon.

Your method takes two colors as arguments, one for filling an one for the outline,
this seems like the best approach to me.

Also there is a Point type that's only used for polygons,
it would be nice to have additional line, circle and rect methods
working on Points instead of x, y pairs.

I think these design flaws would justify a major version update at some point.

Regarding this pull request, can you also add an example for the readme?
Also, the version in shard.yml should be changed to 0.6.0.

[watzon]

Once the 0.35.1 patch comes out for Crystal I'm going to update this and apply fixes for anything that's broken as well.