fixed issue related to checking offset on target rather than only window...

I was having a hard time figuring out why scrollfix was not working in my app, and noticed that the offset is always pulled from `$window`.

This is insufficient as it will not properly work when you have an absolutely positioned container for the main viewport of an app.

I have fixed this by having the offset from the `$target` since it will always default to $window when a parent `ui-scrollfix-target` is not found.

I also have the corresponding passing tests...