Added Polygons

+ Added Lines, Rectangles and Shapes based on Polygons.
+ Added a Demo for Polygons
- Removed unnecessary Demo "Circles.java"

Changes in Code:
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# Rendering now works like the following:
1| Display.smooth(); // Enable AA
2| Display.color(color); // Set color
3| Display.stroke(); // Sets drawmode to stroke.
4| Display.circle(x, y, r); // Draw a circle.
Alternative:
1| Display.harden();
2| Display.color(c);
3| Display.fill();
4| Display.box() / Display.polygon() / Display.rect() / etc...