Added support for children

Usage:

<StatusBarAlert
  visible={true}
  height={68}
  style={{
    padding: 5
  }}
>
  <Image
    style={{ width: 66, height: 58 }}
    source={{
      uri: 'https://facebook.github.io/react-native/img/header_logo.png'
    }}
  />
</StatusBarAlert>