Sortmode option to allow three-state sorting

Would love the jqGrid to support three-state sorting, where columns rotate between ascending, descending, and then unsorted.  It seems like multisort mode may do this, but I have not tried it out.  Single column sort only switches between asc/desc, so there is no way to return the grid to an unsorted state.  That is what this pull request addresses.

Users must request the behavior by setting a sortmode property to 3 in the grid configuration.  By default it is initialized to 2, which gives the previously existing sort behavior.