Add px-auto-complete component

Here is a first pass at the auto complete component.

You can create an autocomplete component that utilizes the flex view and list box for selection items. It also accepts an auto complete source which gives it the list of items to pull from.

## Usage

```html
<px-auto-complete>
  <px-auto-complete-source>
    <ul>
      <li>One</li>
      <li>Two</li>
    </ul>
  </px-auto-complete-source>
</px-auto-complete>
```

I wanted to open this just to get some feedback. I plan on adding a remote source through polymer-ajax as well as cleaning up and making the selection list look better.