fixes #13
It works, but I don't like the presence of

• DrawingBackend::with_tooltips function
  tooltip should be enabled per chart, not backend.
• draw_series_with_tooltips api.
  Ideally, it should work with draw_series API and tooltip should be a configuration parameter.

Challenge:

• The library gives user the control over when to draw. Because of this, we don't have enough information in draw_series to emit the tooltips code.

Alternative API:

• chart.tooltip_mode(TooltipMode) shall enable tooltips for current chart.
  mode can be "Auto", "Always", "Disable". If Always and the backend doesn't support, there should be error.
• On every draw ( and .label, etc) call, keep on building internal state.
  Also, SVG backend shall add classes wherever required to uniquely identify elements, if tooltip mode is enabled.
• On present call, add javascript to enable tooltips.