CSS dependent element width

I made some changes, allowing to avoid fixed element width and rely it on css. I didnt test it a lot, but it works fine in latest Chrome and FF in single and multiple modes.
Usage:
```javascript
$("select.chosen").chosen({
    fixed_width : false
});
```
```css
.chzn-container {
    width: 100%
}    
```
jsFiddle demo: http://jsfiddle.net/UUvQV/1/
P.S. Thanks for all the work, great plugin!