Do not add span8 class to inputs of CheckboxSelectMultiple.

Before:
![with_span8](https://f.cloud.github.com/assets/94636/1322353/82ba4a6e-3428-11e3-81b6-16dbac792326.png)

After:
![without_span8](https://f.cloud.github.com/assets/94636/1322352/82954458-3428-11e3-83f8-842f3a7cc05c.png)