Add IE7-9 support

nprogress currently uses translate3d(), which is only supported on IE10 (along with most modern browsers).

This fix sniffs whether the browser supports translate3d(), and falls back to translate() (IE9 and other non-3D-supporting browsers) or margin (IE7-8) if not.

Adds a new setting (positionUsing) which is populated the first time that NProgress.set() is called, calling NProgress.getPositioningCSS() which determines which positioning CSS rule to use.

Changes NProgress.set() to call barPositionCSS() to determine what CSS to set on the bar (which uses positionUsing to determine whether it uses transform3d/transform/margin).

Sniffing code based heavily on Zynga/scroller IE8-9 support code:
https://github.com/zynga/scroller/pull/4