Support for adding columns dynamically

[leepfrog]

We'd like to be able to add more table columns to our table whenever a user presses a +new button.

It would be nice if this could happen without redrawing the entire table.

[ppong]

@leepfrog I agree that adding table columns shouldn't redraw the table. I will take a look at it.

[ppong]

The computed properties fixedColumns and tableColumns in the tableController should be dependent on 'columns.@each' instead of just 'columns'. I will merge in my fix later

[ppong]

merged