Updated to new skin design

New skin features include:
- Using FontAwesome for icons
- Fresh new look
- Added new MenuButton class. (though not in skin yet)
- Created SliderHandle class, which subclasses VolumeHandle and SeekHandle

Code changes that break the old skin:
- Killed loading spinner icon javascript. Now relies on CSS.
- Added contentEl to vjs.Menu. There's an extra div around menus now, which allows for better placement.