Fix clear button positioning (single select mode).

Add a containing div for buttons that is floated right so that we can
correctly order the buttons as desired using DOM positioning, etc.
Make single-select clear button key accessible.

Add tab index so that single-select clear button can receive focus and
listen for enter key press. Additionally, click or keypress should place
focus back in the input field.
Merge pull request #25 from benbasson/fix-clear-button

Fix clear button positioning and behaviour.