Fixes header cell sorting classes for BackbonePageable collections

[devonreed]

I noticed a couple of problems with the Backgrid header CSS classes w/r/t sorting Backbone.PageableCollection instances. These problems included:

• multiple columns being marked as the active sorting column (when there should have only been one marked as active)
• sorting-related CSS classes being cleared when events outside of Backgrid caused collection sort events (specifically getNextPage or getPreviousPage)

This PR addresses these problems.