using computed width instead of content width to give users flexibility to change box-sizing

This commit does not change anything for most users -- those who do not specify box-sizing for the token-list `ul` or who specify `box-sizing: content-box`. It just gives users the flexibility to add left/right padding to the token list `ul` and set `box-sizing: border-box` on it and the dropdown, so that their widths will still match, despite the padding.

Note that `.css("width")` is also [a performance win](http://blog.jquery.com/2012/08/16/jquery-1-8-box-sizing-width-csswidth-and-outerwidth/) over `.width()`.