Python exception

```
import darm
d = darm.disasm_armv7(0)
print d.Rd, d.Rn, d.Rm, d.shift
print d
```

```
r0 r0 r0 Traceback (most recent call last):
  File "fuck.py", line 5, in <module>
    print d.Rd, d.Rn, d.Rm, d.shift
  File "darm.py", line 97, in __str__
    type_name = self.type_name()
  File "darm.py", line 94, in type_name
    return 'S_' + _lib.darm_shift_type_name(self.type_)

```
