feat($cordovaAdMob): add promises

- fix documentation links
- add promises for methods
- add unit tests

*BREAKING CHANGE:*

to migrate follow the following steps.

Before:

```
$cordovaAdMob.createBannerView({ exampleOption: 1 }, function () {
  // success
}, function () {
  // error
});
```

After:

```
$cordovaAdMob.createBannerView({
  exampleOption: 1
}).then(function () {
  // success
}, function () {
  // error
});
```

resolves #243