feat($cordovaBatteryStatus): bringing the events in a angular way

- adding tests
- $on implementation

BREAKING CHANGE: bringing the events in a angular way

To migrate the code follow the example below:

Before:

$cordovaBatteryStatus.onBatteryStatus(function(result) {
  var batteryLevel = result.level;       // (0 - 100)
  var isPluggedIn  = result.isPlugged;   // bool
});

After:

$cordovaBatteryStatus.$on('batterystatus', function (result) {
  var batteryLevel = result.level;       // (0 - 100)
  var isPluggedIn  = result.isPlugged;   // bool
});
Merge remote-tracking branch 'upstream/master' into feat-battery-status-angular-events
test($cordovaBatteryStatus): add more tests
Merge pull request #297 from mrzmyr/feat-battery-status-angular-events

feat($cordovaBatteryStatus): bringing the events in a angular way