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

adamkundrat/angular2-google-maps,pSuwalski/angular-google-maps,staticint/angular2-google-maps,rafalsobota/angular2-google-maps,pSuwalski/angular-google-maps,staticint/angular2-google-maps,SebastianM/angular-google-maps,rafalsobota/angular2-google-maps,adamkundrat/angular2-google-maps,tobper/angular2-google-maps,eques/angular2-google-maps,pSuwalski/angular-google-maps,eques/angular2-google-maps,rafalsobota/angular2-google-maps,tobper/angular2-google-maps,tobper/angular2-google-maps,SebastianM/angular2-google-maps,rc3media/angular2-google-maps,rc3media/angular2-google-maps,rc3media/angular2-google-maps,SebastianM/angular2-google-maps,eques/angular2-google-maps,adamkundrat/angular2-google-maps,staticint/angular2-google-maps,SebastianM/angular-google-maps,caravancoop/angular2-google-maps,caravancoop/angular2-google-maps,caravancoop/angular2-google-maps,SebastianM/angular2-google-maps