Added the ability to scroll horizontally in the stretch data grid. Resolves Issue 159

Resolves #159
Resolves https://fuelux.uservoice.com/forums/181290-general/suggestions/3624683-datagrid-should-be-able-to-hide-show-columns

Added the ability to scroll horizontally on the datagrid in stretch mode. Just setting a width on the datagrid is enough to enable this.