add apple-touch-icon

add apple-touch-icon for better mobile support

HTML5 Boilerplate implementation.
See:
https://github.com/h5bp/html5-boilerplate/blob/master/src/index.html#L10
https://github.com/h5bp/html5-boilerplate/blob/master/src/apple-touch-icon.png

![bower-app-icon](https://cloud.githubusercontent.com/assets/5244986/5569189/4cec18d6-8f6c-11e4-81e0-ec4a631cb607.png)