fix corrupt child nodes in IE9 and below

- to reproduce problem, click any date inside the custom range picker in
  IE9 and below
- this fixes outerClick() incorrectly hiding the datepicker when various
  elements inside get clicked (<td> elements and next/prev buttons)
  - .closest() returns nothing when .html() is used by itself to remove
    subtrees of the DOM. .empty() will apparently keep the removed
    structure in tact for methods like .closest()
- see 'Note' section under the .html() docs for quick explanation:
https://api.jquery.com/html/#html2
Merge pull request #290 from jakswa/js/ie9-dom-removal-fix

fix corrupt child nodes in IE9 and below