Use pageXOffset as fallback

IE9 & IE10 do not have the alias scrollX/Y
pageXOffset seems the original and standard property. See https://developer.mozilla.org/en-US/docs/Web/API/Window.scrollX

Maybe it could be used alone, not as a fallback for scrollX/Y
Merge pull request #56 from JSteunou/issue-55

Use pageXOffset as fallback