Add examples and refactor code

Add examples and refactor code

elements/Circle.js

@@ -0,0 +1,25 @@
+import React, {
+    Component,
+    PropTypes
+} from 'react-native';
+import Ellipse from './Ellipse';
+
+let propType = PropTypes.oneOfType([PropTypes.string, PropTypes.number]).isRequired;
+class Circle extends Component{
+    static displayName = 'Circle';
+    static propTypes = {
+        cx: propType,
+        cy: propType,
+        r: propType
+    };
+    render() {
+        return <Ellipse
+            {...this.props}
+            r={null}
+            rx={this.props.r}
+            ry={this.props.r}
+        />
+    }
+}
+
+export default Circle;