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 ). 
I'm no expert at pushState but it seems to update the URL when you add another field. However, the back button doesn't work. It would be possible to store the params and have them apply when you hit the back button, no?