Select date component

Adds a {{eui-selectdate}} component that functions like the select component, but for dates. It can be used to select a single date, or a date range. I have already pushed the documentation changes to the website repo so you can review the api there as well and try it out.

Also adds a new mixin which isolates the modal-like behaviour for reuse. Right now only modal and selectdate uses it, but i am going to look into refactoring poplist so both select and dropbutton could make use of it.

This adds twix.js as a dependency for formatting the dates shown on the selects.

I have also added the jqueryUI position function as a utility. The selectdate makes use of it for its positioning, in the future both the poplist and the modal should also use it. 

Thoughts? cc @elucid @ghedamat