Remove forced widths in order to get responsive dropdown menus

As the title suggests, I've removed the forced widths used when creating chosen objects. Now the width of the select menus depends on their wrapper.

I've uploaded it to `gh-pages` so you can see it working: http://elboletaire.github.io/chosen/

Obviously, resize the window to see it in action :smiley: 
if ``this.container_width()`` is not used anymore, it should be removed