Fixing initialization declaration

Changed self to super, otherwise, will call init constructor from
subclasses and can stay in infinite loop.

Ex:

```Objective-C
@implementation SignUpViewController

- (id)init {
    self = [super initWithUrl:[NSURL URLWithString:kSignUpUrl]];
    return self;
}
```

It will call:
```Objective-C
- (id)initWithUrl:(NSURL*)url {
    self = [self init];
    if(self)
    ...
}
```

That will call SignUpViewController.init