try to fit values in charts above bars

* use the same font for y axis as for x
* use the same or smaller font for values as for x axis' labels
* try to fit values labels horizontally or vertically
* if failed then hide
* use feature in newer GD::Graph to avoid hiding all labels
drop shebang from t/ file
return '0 seconds' for ->DurationAsString(0)

This is backwards compatible value that was broken by recent changes.
I don't know better value to suit most use cases, so stick to what
we had before.
adjust test to recent changes in Date.pm