Augmented assignment leads to type complaints

Hi,

I am using mypy 0.4.4. It seems augmented assignments aren't yet dealt with correctly.

```
weight0 = 65.5
weight0 = 65
weight0 *= 0.5                  # Gives error, unsup operand types for * ("int" and "float")
```

```
weight1 = 65.5
weight1 = 65
weight1 = weight1 * 0.5    # Gives no error
```
