Fixes for mobile interaction in typeahead mode.
Fixes for mobile interaction in typeahead mode.

* Added CSS to prevent rapid subsequent clicks from selecting the text in the `input` field when disabled (i.e. not in `typeahead` mode).
* Replaced previous dropdown positioning workaround with `no-overlap` attribute that was subsequently documented.
* Added `on-touchend` handlers to prevent ghost clicks from bleeding through the dropdown to input fields, etc. below.
* Fixed the direction icon toggling binding to `_dropdownIcon()`.
* Refactored `_onCaptureTap` so that we're not fighting against `iron-dropdown`'s capture method.
    * Added `no-cancel-on-outside-click` to `iron-dropdown` instance.
    * Our version just blurs the input and hides the dropdown if the "tap" occurs outside our element's DOM tree.
    * Changed the lifetime of that listener to be as long as this element is attached rather that just when the dropdown is open.
        * Otherwise the input would only be notified to blur if the dropdown was open.
* Added `_onInputFocus()` handler because software keyboards on mobile devices include alternate ways to focus an item beyond taps (form navigation buttons at the top of the software keyboard).

jasongardnerlv/radium-combo,johnyanarella/radium-combo