+= operator not converted properly

``` java
public class Test {
    public void m() {
        int x = 5;
        x+=1;
    }
}
```

gets converted to

``` java
class Test {
  def m(){
    var x=5
    x+=1
  }
}
```

which is not valid xtend code (because of +=). It should convert into
x = x +1
This is also affecting Strings, not only numbers and is the case for other operators as well
(in particular, x |= 8; )
