Change check for `is_bar`. Now just compare to BarContainer objects.

This also removes `is_barh`. Handling of orientation is postponed
to the actual drawing of the bar traces.
Added logic to add bars with new `is_bar` test.
removed print statement for debugging.
add check to see if we should 'stack' the bar groups.
Added `hovermode='x' (or 'y')` when stacked bar charts are present.
Merge pull request #95 from plotly/iss87

Iss87