add basic attribute support for visible and sortable - possible fix for ...

fix for #40 - disable sorting of columns.
Added two new attributes:
[DataTablesSortable] - set to false to prevent sorting
[DataTablesVisible] - set to false to make column hidden.

Note: because the bSaveState is set to true in the cshtml file you need to clear the browser cookie/local storage in order for this to work.  Reading the dataTables documentation, this seems to be by design.