Using one tooltip per chart rather than per series
Updated comment after adding initializeTooltip to Chart.
Typo: offest => offset
Updated comment: Series => chart.
Allowing series to have different tooltip offsets

The tooltip offset needed to be set on mouseover rather than when the
series is drawn. This means that the positionTooltip function is not
needed.
Removed tests for Series.initializeTooltip
Merge pull request #191 from niksoper/single-tooltip

Using one tooltip per chart rather than per series