Bug 905974 - [bb][refactor] Switches

I've created 2 skin files: `skin.default.css` and `skin.check&radio.css`.
I think the best approach is to treat the `on/off` switch as the default apperance and then generate that skin file for the radio/checks ones. In that way we avoid to over-defining classnames in the elements like `bb-switch switch` or `bb-switch radio danger`.

I will like to remove some images in order to achieve a more "pure" css solution. But i'm wating to confirm it with @ because a possible performance impact in long element lists.