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-20141209-004032](https://cloud.githubusercontent.com/assets/207895/5349720/1af0e428-7f3c-11e4-9b70-b70babe5456c.jpg)