Cast stats counts as integers (bug 997465)

Also:
- use a setting for the collector addon
- remove dbg from stats/manager.js

Ref: https://bugzilla.mozilla.org/show_bug.cgi?id=997465
Good question, but AFAIK we only deal with integers, as we are doing Sum() of an `integer` field / db column (`count = models.IntegerField()`).