added new settings

topPositionCorrelation - (int) will be plused to top position if droplist will be show at the top;
bottomPositionCorrelation - (int) will be substracted from top position if droplist will be shown at the bottom;
hideOnWindowScroll - (bool) if false then showed droplist will not hide on window scroll.
Merge pull request #134 from eliotik/patch-1

added new settings topPositionCorrelation, bottomPositionCorrelation, hideOnWindowScroll.