Use label instead of badge for boolean elements as per ...

A solution for sferik/rails_admin#2131 may be to use `label` instead of badges, the result is a little less rounded.

Up to you guys.

![screenshot-20141209-003409](https://cloud.githubusercontent.com/assets/207895/5349646/42bb0bf6-7f3b-11e4-821b-3d1fcb6d464c.jpg)