Ensure container is always rendered in front

The default behavior of the autocomplete widget was to render it behind other form elements on the page.

![autocomplete-bug](https://f.cloud.github.com/assets/442971/1525313/cf794e64-4bd3-11e3-8c6b-21c1f8c09118.png)

This PR sets the z-index of the autocomplete container higher so it should be displayed in front of other elements.