Expand .str to DataFrame
#### Code Sample, a copy-pastable example if possible

```python
mask = df.str.match(re)
AttributeError: 'DataFrame' object has no attribute 'str'
```
#### Problem description

I always run into this. DataFrame has no method str. Though I should be easy to map this method to the Series.str  and call it for each Series. Is there any reason why this is not implemented, other than no one did it yet? 

