don't use iCheck in bootstrap button group

prevent iCheck on bootstrap 3 button groups

before:
![bildschirmfoto 2014-05-04 um 11 28 29](https://cloud.githubusercontent.com/assets/241080/2872663/81970f86-d370-11e3-86a8-898b58350f04.png)

after:
![bildschirmfoto 2014-05-04 um 11 41 36](https://cloud.githubusercontent.com/assets/241080/2872664/83b8bd1e-d370-11e3-9ee9-33a2c65040ab.png)