Fixed math for colborder class within grid erb.  Resolves Lighthouse Issue #60 (http://blueprintcss.lighthouseapp.com:80/projects/15318/tickets/60-1-pixel-column_width-change-breaks-sample-page-layout)
Fixed math for colborder class within grid erb.  Resolves Lighthouse Issue #60 (http://blueprintcss.lighthouseapp.com:80/projects/15318/tickets/60-1-pixel-column_width-change-breaks-sample-page-layout)

shinphoowin/blueprint-css,sagax/_blueprint-css,chriseppstein/blueprint-css,tladesignz/blueprint-css,shinphoowin/blueprint-css,blueprintcss/blueprint,baiyanghese/blueprint-css,cutefrank/blueprint-css,cofffeee/test,sagax/_blueprint-css,cutefrank/blueprint-css,joshuaclayton/blueprint-css,beppec56/blueprint-css,beppec56/blueprint-css