Grid.unselectSelectedModels

Uses the select-all checkbox to deselect all selected models. Factor out separate function getSelectAllHeaderCell.

I found that I needed a function to unselect all currently selected models. In my case it was after the user performs an action on some selected rows.

##### Testing

One test is added. The same set of tests fail on this branch as fail on wyuenho/backgrid master.

![image](https://f.cloud.github.com/assets/52205/345083/16380bb8-9e16-11e2-9b97-da5515128aee.png)

The added test is not DRY. I could incorporate it into the test for `getSelectedModels` if you prefer.