Add support for default value to $bind.

I found myself writing some boilerplate code over and over to set a default value if the initial loaded value was not set. I was originally using $on('loaded', ...) but rewrote it to fit better into the $bind function.

This changes the following:

```javascript
var fb = $firebase(new Firebase('http://myapp.firebaseio.com/'));
$scope.stuff = {};
fb.$bind($scope, 'stuff');
fb.$on('loaded', function(value) {
  if (value == null) {
    $scope.stuff = { x: 123 };
  }
});
```

To:

```javascript
var fb = $firebase(new Firebase('http://myapp.firebaseio.com/'));
$scope.stuff = {};
fb.$bind($scope, 'stuff', function() {
  return { x: 123 };
});
```