DateOffset should have a property for a "normalized" shortcut string
Following discussion in https://github.com/pydata/pandas-datareader/pull/257

`DateOffset` should have a property for a "normalized" shortcut string

So for example we should have

```python
to_offset("7Min").normalized_string == "7T"
to_offset("7MIN").normalized_string == "7T"
to_offset("7min").normalized_string == "7T"
```

Maybe something like

```python
@property
def normalized_string(self):
     if self.n == 1:
         return self.rule_code
     else:
         return str(self.n) + offset.rule_code
```