Added icon support for Tabs

Reused code from `settings` shortcode to display a small icon beside the tab title. The user can either upload an icon or use fontawesome icons to precede the text. This has also been added to the tabs.example file. Preview of final output below.

![tabs-with-icons](https://cloud.githubusercontent.com/assets/2360912/5430905/e1ba602a-8441-11e4-9cb1-f7975b40c1c5.PNG)