Issue : #14

Reason - Once coachmarks are displayed I was setting last and current visible to null
but not _visibleTooltips and when user wants to show new coachmarks
I was setting _visibleTooltipIndex to 0 and setter of this variable was again assigning last coachmark's 0th item to current and next iteration was setting it to last visible

Solution - rest _visibleTooltips once we are setting up new values