Added Linear-Gradient mixins for Opera 11 and Chorme 11

[VoQn]

Hi, madr

I'm your "css3-sass-mixins" user, always using your sass library when develop Web Application with Sinatra. :-)

I forked your repository. And added linear-gradient mixin for Opera 11. and -webkit-linear-gradient (Enable Currently Webkit and Chrome)

see example.

-o-linear-gradient(#000, #fff); /* of course gradient #000 to #fff */

/* from Currently Webkit(and Chorme 11), instead of 
 * too many argument vendor prefix.
 * like -> "-webkit-gradient(top, left top, right bottom, color-stop(1, $from),  ...)" */
-webkit-linear-gradient(#000, #fff); 

please pull my commit to your master. ;-)

[madr]

Awesome work! :D