[fix bug 984499] Update Sandstone Less mixins for CSS gradients

* Updated CSS gradient syntax for mixins in `lib.less` and `mixins.less`
* Removed `-khtml` vendor prefixes.
* Removed the outdated `-webkit-gradient` syntax rules.
* Removed `-ms` vendor prefixes (I believe these we're only used in IE10 Preview. The release version uses standard syntax).

I've left in `-moz` to legacy Firefox support, but I'm totally open to what we include or leave out. Up for discussion!