Changes to column summation to allow skipping columns

I'm not sure if this was the intended functionality or not, but I changed the getSumRow function of SummedColumnBody to loop through all columns instead of columnsToSum. This way, empty td elements will be inserted and you'll end up with a full row instead of a row that only has cells for the items in columnsToSum.

I also removed the reference to columnsToIgnore, because it was causing issues and didn't seem to actually do anything.

To test, I changed line 35 of example.html from
body: window.Backgrid.SummedColumnBody.extend({ columnsToSum: ['name', 'value'] })
to
body: window.Backgrid.SummedColumnBody.extend({ columnsToSum: ['value'] })

In the previous version, the summation for "value" would show up under the "name" column.