Skin: Extend CSS3 gradient support

Add CSS3 gradient support for the following browsers:
- Chrome, Safari 4+
- Chrome 10-25, iOS 5+, Safari 5.1+
- Firefox 3.6-15
- Opera 11.10-12.00
- Chrome 26, Firefox 16+, IE 10+, Opera 12.10+

Renamed parameters 'from' and 'to' to 'start' and 'end' so they don't get mixed up with the
property 'to bottom' introduced in the prefix-free linear-gradient.