Update linear-gradient, border-radius and box-shadow CSS properties

Reduce by 21% CSS size and increase compatibility with newest browser (replacing deprecated "bottom" by "to top") as a step toward minfication (https://github.com/sebsauvage/ZeroBin/issues/60)

Up-to-date syntax:
https://developer.mozilla.org/en-US/docs/Web/CSS/linear-gradient
https://developer.mozilla.org/en-US/docs/Web/CSS/box-shadow
https://developer.mozilla.org/en-US/docs/Web/CSS/border-radius

Overall support: 100% of updated browsers, and 73-82% of all existing browsers (even obsolete ones):
http://caniuse.com/border-radius
http://caniuse.com/css-boxshadow
http://caniuse.com/css-gradients