Create Example using Code from Blog Post

[cancan101]

The blog post does a better job of demonstrating styling aspects of ex-navigator than the examples in the REDAME. I suggest either updating the README or creating a working example using the code in the blog post.

For example:

    renderLeftButton(navigator) {
      return (
        <TouchableOpacity
          touchRetentionOffset={ExNavigator.Styles.barButtonTouchRetentionOffset}
          onPress={() => navigator.parentNavigator.pop()}
          style={ExNavigator.Styles.barLeftButton}>
          <Text style={ExNavigator.Styles.barLeftButtonText}>Done<Text/>
        </TouchableOpacity>
      );

uses a number of styling and offset constants that are not documented in the README.

[ide]

Opened #18 for people interested in contributing.