#979 - Calculate with with window.getComputedStyle rather than outerWidth

Using `window.getComputedStyle` to compute the width for `.chzn-container` divs takes out the possibility of a rounding error if a `select` element's width is computed to precision of less than a pixel. I.e., if a select has `width: 126.5px` specified, the `.chzn-container` will take on a width of `127px`. This can create divs which are marginally too wide.

The code on this branch should fix the issue.