Enhanced multiSelect

I couldn't rebase #693 so here's the updated version

Some enhancements to the multiSelect jquery widget as used in settings/users. Refs owncloud/apps#244 and #971

Changes from original:

* Previously only used the underlying options values, not the text.
* Can now be used as singleSelect like Eric Hynds jquery.multiselect
* If there's not enough room in the viewport the widget will pop up instead of drop down.
* Uses both items in the checked array argument and sets those as selected, and adds items to the checked array that are selected.

And in this version:

* Added sort and having selected items first options.
* Made max-height dynamic instead of hardcoded to 20em.
* Don't try to show the dropdown above button if there's less space than below.

/cc @jancborchardt @icewind1991