Column attribute support & Header row sort support

 To add support for the following extensions; Reorderable columns, Grouped columns and (re)sizeable columns I propose a few small changes to backgrid.

These are:
- 'attributes' support on column definition. Maybe we should limit this using [pick](http://underscorejs.org/#pick) to allow only valid html5 attributes. This is one way to support 'colspan' and 'rowspan' on columns.
- Header rows did not update after a sort, now they do