added ellipse

docs/lib-ellipse.md

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+# ellipse
+
+Creates an ellipse. 
+
+## Parameters
+
+- `axes` : 2 element vector `[x, y]` where x is the semi-major axis and y is the semi-minor axis.
+- `$fa`, `$fs`, `$fn` : The shape created by this module can be viewd as `resize([x, y]) circle(r = x)` (but not the real implementation inside the module) so you can use these global variables to control it. Check [the circle module](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Using_the_2D_Subsystem#circle) for more details. 
+
+## Examples
+
+	include <ellipse.scad>;
+	
+	ellipse([40, 20]);
+
+![ellipse](images/lib-ellipse-1.JPG)
+
+	include <ellipse.scad>;
+	
+	$fn = 8;
+	ellipse([20, 40]);
+
+![ellipse](images/lib-ellipse-2.JPG)
+

src/ellipse.scad

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+
+/**
+* ellipse.scad
+*
+* Creates an ellipse.
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-ellipse.html
+*
+**/
+
+module ellipse(axes) {
+    frags = $fn > 0 ? 
+        ($fn >= 3 ? $fn : 3) : 
+        max(min(360 / $fa, axes[0] * 2 * 3.14159 / $fs), 5);    
+    
+    step_a = 360 / frags;
+    polygon(
+        [
+            for(a = [0:step_a:360 - step_a]) 
+                [axes[0] * cos(a), axes[1] * sin(a)]
+        ]
+    );
+}