Fix #285 ensure for formatters in the NumberCell and DatetimeCell classes are instances at definition time
Fix #285 ensure for formatters in the NumberCell and DatetimeCell classes are instances at definition time

jesseditson/backgrid,getjaco/backgrid,qferr/backgrid,qferr/backgrid,goodwall/backgrid,bryce-gibson/backgrid,ludoo0d0a/backgrid,blackducksoftware/backgrid,bryce-gibson/backgrid,goodwall/backgrid,getjaco/backgrid,kirill-zhirnov/backgrid,wyuenho/backgrid,kirill-zhirnov/backgrid,ludoo0d0a/backgrid,digideskio/backgrid,digideskio/backgrid,blackducksoftware/backgrid,crealytics/backgrid,wyuenho/backgrid,jesseditson/backgrid,aboieriu/backgrid,aboieriu/backgrid