DatetimeIndex.resolution throws NameError
Attempting to retrieve the resolution attribute throws this exception:

``` python
34    @classmethod
35    def get_str(cls, reso):
36        return {RESO_US : 'microsecond',
37                    RESO_SEC : 'second',
38                    RESO_MIN: 'minute',
NameError: global name 'RESO_US' is not defined
```
