Make sure that box-sizing is set to content-box

[timeu]

Some 3rd party CSS stylessheets (i.e. Bootstrap) set box-sizing on all elements to border-box.
This will lead to display problems.

The current workaround is to set the box-sizing of all descendant elements to content-box.

[jasonlemay]

I'd rather like to update Chosen to play well with border-box instead. So much more easier to handle and style this way.