added BSModalDatePicker

PR for #1

this might be worth a totally different repo and a pod.

```objc
BSModalDatePickerView *picker = [[BSModalDatePickerView alloc] initWithDate:[NSDate now]];
picker.mode = UIDatePickerModeDate;
[picker presentInView:self.view withBlock:^(BOOL madeChoice) {
  if (madeChoice) {
    NSLog(@"You selected date %@", picker.seletedDate);
  } else {
    NSLog(@"You cancelled the picker");
  }
}];
```