negative operator for descending order
- 08b3c0ee03576e16965b9003b38b4bad4b126fb3
- 3ced2b6d3955243fb815250158c44af94b212781
- 2f114136cb823f5a5b0c936dcc93d313c9e4f368

I'd like to propose a syntax sugar for ordering. Instead of 

```
.order_by(User.name.desc())
```

one can also do

```
.order_by( - User.name)
```

which simply requires a hook method in class Node:

```
  def __neg__(self):
         self._ordering = "DESC"
```
