Better support for options with empty values

This PR addresses two separate issues.

1) If an `<option>` does not have an explicit `value` attribute, selleckt does not parse it correctly. See doc at http://www.w3.org/wiki/HTML/Elements/option#HTML_Attributes

2) As would be expected, if an option with an empty value is provided, selleckt displays a placeholder when said option is selected. If the underlying select tryggers a change event for that option though, the placeholder is lost.

Both issues are illustrated here: http://codepen.io/codazzo/pen/wnFsK
The solution can be seen working here: http://codepen.io/codazzo/pen/ErJcm