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 ). 
It makes for a clearer separation between the control tools and the actual content. Well, at least I like better with than without, but that's clearly a graphical preference.