Consider adding Size-auto

https://github.com/donquixote/bootstrap-autocolumns/blob/master/bootstrap-autocolumns.css
example of usage:

```
<div class="row">
  <div class="col-xs-auto">Left</div>
  <div class="col-xs-auto-right">Right</div>
  <div class="col-middle">Middle</div>
</div>
```

fiddle: http://jsfiddle.net/9wzqz/
