Solve Issue #163 to IE9

Solve Issue #163 to IE9

Aparently, IE9 has a problem with getComputedStyle function to the property backgroundPosition.
Fortunately, IE9 has an alternate property (currentStyle) with the "same" functionality that getComputedStyle.

I only add a hack to get currentStyle when is available instead getComputedStyle and check that currentStyle return a value (some times on IE10 this property not return nothing).