fix(SebmGoogleMap): show map when zoom is not set

This fixes the following (allowed) use of SebmGoogleMap.
When the zoom was not set, the map was empty.

```
<sebm-google-map [latitude]="lat" [longitude]="lng">
</sebm-google-map>
```

closes #81

src/directives/google-map.ts

@@ -96,7 +96,8 @@ export class SebmGoogleMap implements OnChanges,
  }
 
  private _initMapInstance(el: HTMLElement) {
- this._mapsWrapper.createMap(el, {center: {lat: this._latitude, lng: this._longitude}});
+ this._mapsWrapper.createMap(
+ el, {center: {lat: this._latitude, lng: this._longitude}, zoom: this._zoom});
  this._handleMapCenterChange();
  this._handleMapZoomChange();
  this._handleMapMouseEvents();
@@ -118,7 +119,7 @@ export class SebmGoogleMap implements OnChanges,
  * Sets the zoom level of the map. The default value is `8`.
  */
  set zoom(value: number | string) {
- this._zoom = this._convertToDecimal(value);
+ this._zoom = this._convertToDecimal(value, 8);
  if (typeof this._zoom === 'number') {
  this._mapsWrapper.setZoom(this._zoom);
  }
@@ -140,13 +141,13 @@ export class SebmGoogleMap implements OnChanges,
  this._updateCenter();
  }
 
- private _convertToDecimal(value: string | number): number {
+ private _convertToDecimal(value: string | number, defaultValue: number = null): number {
  if (typeof value === 'string') {
  return parseFloat(value);
  } else if (typeof value === 'number') {
  return <number>value;
  }
- return null;
+ return defaultValue;
  }
 
  private _updateCenter() {