Support empty first series in stack

When using the RangeSlider on a graph with data of different lengths of series, it can easily happen that the displayed region is one where some of the series have no data.

If this happens to be the first series, then Rickshaw will barf, as it's internal domain() calculation cannot cope with that.

This patch expands the power of the domain() calculation and adds some lenience into the validation routine of Rickshaw.Graph to make it easier to unit test that situation.

This pull request probably obsoletes shutterstock/rickshaw/pull/244 and closes shutterstock/rickshaw/issues/122.

Do you like it?

Do you want any changes in implementation to accept this?

Best regards,
Martin