from issue2 about pushState

from the issue : https://github.com/alicelieutier/smoothScroll/issues/2

changed the followings
- not to pushState same history
- activate :target in Google Chrome
- the way to get an element's position(getTop)

By the way, CSS :visited doesn't work in IE(9) when the original event is prevented (I'm using Vista, so I can't test on IE 10).