Add ajMatchmedia directive

This adds a `<aj-matchmedia>` directive. Please note:

* As per the best practice described in the [AngularJS Directives Developer Guide](https://docs.angularjs.org/guide/directive#creating-directives), I chose `aj` (from AnalogJ) as the prefix.
* The `on` attribute is given priority over the `is` attribute.
* The aliases for both `on` and `is` are supported.
* With the `on` attribute present the supplied callback is called when the media query is evaluated again.
* With the `is` attribute present the result of the media query evaluation is being fed into `queryMatches`.

Example of usage:

```html
<aj-matchmedia is="isTablet" query-matches="isTablet"></aj-matchmedia>
<div ng-show="isTablet">Content is shown when isTablet is true on pageload</div>
<aj-matchmedia on="onTablet" query-listener="onTablet(mediaQueryList)"></aj-matchmedia>
```

This fixes #8.