Fixes bug 1067196 - Added columns choice to the Reports tab of signature report.

@peterbe r?

What's cool is that the columns were already supported, all I had to do was enable them in the HTML/JS. Hence the fact that there is no unit test added here: they already exist (see https://github.com/mozilla/socorro/blob/master/webapp-django/crashstats/signature/tests/test_views.py#L144 ). 
Maybe it's too late to mention this but I really don't understand what the point of this `hr` is. It's just a strange black line between the fields and the table.