Percentage Width Support

There have been a number of requests for variable width support in Chosen, but none have adequately solved the problem.

This patch allows users to pass in a 'width' option.  I've done cross-browser compatibility testing and verified that this works properly on FF8, Chrome, Safari, and Opera (IE is unchecked).

I accomplished this by using absolute positioning on the search input, results, and choices, with a 100% (css customizable) width for the drop-down itself.  This has the added benefit of removing some manual width calculations in the implementations.

I pushed a modified gh-pages (with % container width) that demonstrates the result : http://justindujardin.github.com/chosen/