[change] Image 'source' dimensions and RN layout

Adds support for 'width' and 'height' set via the 'source' property.
Emulates RN image layout (i.e., no dimensions by default).

Fix #10

examples/components/Image/ImageExample.js

@@ -218,7 +218,7 @@ const examples = [
     render: function() {
       return (
         <Image
-          source={{uri: 'http://facebook.github.io/react/img/logo_og.png'}}
+          source={{ uri: 'http://facebook.github.io/react/img/logo_og.png', width: 1200, height: 630 }}
           style={styles.base}
         />
       );