Keep tooltip in div with position relative on selector
Keep tooltip in div with position relative on selector

Before, the (absolute) tooltip was not contained within the selector
div. Test by setting ` style="margin-top: 200px”` on `<div
class="container">` in `example/index.html`. Now fixed by setting
position: relative on selector.
Side effect was that tooltip came closer to selected cell, so updated
position.

DKirwan/calendar-heatmap,DKirwan/calendar-heatmap