Improvements for Bootstrap 3.0 support

I've done some improvements for Bootstrap 3.0 support. Including:
    
- Fixed problem with inputs of type file, that were displayed inside input text elements
- Added support for Django Form `required_css_class` and `error_css_class`
- Added CSS class `multiple-checkbox`, to be used to easily hide `<li>` elements in multiple checkboxes from your styles file. E.g:

```
.multiple-checkbox ul { /* Used to hide <li> elements in multiple checkbox forms */
  padding-left: 15px;
  list-style: none;
}
```