Properly fix tooltip x/y with respect to current viewport

A more complete version of the tooltip position fixups.  It fixes both x and y co-ordinates now, and does this with respect to the current viewport, so a large page that's scrolled down to see the graph(s) will still work.

Hopefully I got the pull request all correct this time :).

NB: This is still somewhat of a heuristic.
What I'd like to do is something like:

var totalTipWidth = $("#flotTip").width() + this.tooltipOptions.shifts.x + $("#flotTip").css('padding-left') + $("#flotTip").css('padding-right') + $("#flotTip").css('border-left-width') + $("#flotTip").css('border-right-width');

But the .css() properties don't seem to be set at this point (they
return NaN).  So instead there's the heuristic of using multiples of the
shifts.x/y instead.

The change in the call to that.updateTooltipPosition() is from:

https://github.com/Cu3bluKekc/flot.tooltip/commit/f0b6b859dadedad64a08e178b825dce0089a5c0c

who first prompted me to look at the y fixup as well.