banner should be appended after the css is compressed

currently if you have a newline at the end of the banner option:

```
less: {
  options: {
    compress: true,
    banner: '/*! Banner */\n'
  }
}
```

the trailing newline will also be removed. the banner should not be appended before the css is compressed. it should let users to choose whether the banner is in a new line or not.
