[Polylines] Init polylines directive

Add Polylines to AngularGM:

- [x] Display polyline(s) on AngularGM maps
- [x] Google configuration options via fn or obj in directive
- [x] Error/sanity checks
- [x] Watch $scope to remove lines from gm-map
- [ ] Support Google Map events
- [x] API documentation

```html
<gm-map gm-map-id="'myMap'" gm-map-options="mapOptions">
        <gm-polylines gm-objects="polylines"
                      gm-get-path="object.path"
                      gm-get-polyline-options="getPolylineOptions(object)">
        </gm-polylines>
</gm-map>
```

```javascript
$scope.mapOptions = {yada: 'yada'};

$scope.getPolylineOptions = function(line) {
    return {
        strokeColor: '#F00',
        strokeOpacity: 1,
        strokeWeight: 2
    }
}

$scope.polylines = [
    {
        path: [
            {lat: 37.772323, lng: -122.214897}
            {lat: 21.291982, lng:-157.821856},
            {lat: -18.142599, lng: 178.431},
            {lat: -27.46758, lng: 153.027892}
        ],
        otherDomain: 'specific logic',
        hello: 'world'
    }
];
```

I tried to follow your pattern with how you did the markers. There may be a better way than to have path as an array of lat/lng objects. Would appreciate some feedback while I continue to work on this.