Add selection object to select event.
Add selection object to select event.

This gives the developer access to the relative x/y positions of the
corners of the selection.  Necessary for axes other than the default
axis.  For #100.

jessiejea/Flotr2,beastlike/Flotr2,jessiejea/Flotr2,alanponce/Flotr2,alanponce/Flotr2,alanponce/Flotr2,nicgallardo/Flotr2,RobertoMalatesta/Flotr2,nicgallardo/Flotr2,beastlike/Flotr2,HumbleSoftware/Flotr2,RobertoMalatesta/Flotr2,beastlike/Flotr2,HumbleSoftware/Flotr2,jessiejea/Flotr2