IE fixes and alt-arrow support

alt-down/up is the standard way of opening a select when using a
screen reader. This also fixes the gradient in IE, so now it
displays the right colors. Also added a note about Placeholders.js,
so the placeholders on multi-select boxes will work in IE 9 and
below.
Fixed string to object conversion
Support duplicate options

Now that the data objects have the `element` property, we can test
to make sure it's a DOM element and then use it for selecting the
option. This allows us to select multiple options with the same id,
as well as handle cases where that is already happening.

You cannot use `$e.val()` to select two options with the same id,
as jQuery will reject it, but you can set the second option to
`.selected = true`, which is supported.