Fix for scroll offset in Util.getPosition()

Previously, Util.getPosition didn't deal with scrolling
particularly well.  This fixes that by calculating the
scroll offset when dealing with getting mouse positions.

Credit to @erikgull and @emmar for the initial version of
the fix.  Credit to Brian Huismanfor the initial code.

Closes #295
Relevant to #258
Better parsing of query string variables
Merge pull request #363 from kosmasgiannis/query_parse

Better parsing of query string variables