encode/decode method for string module (like in np.char.encode )

This is already in included in numpy but for convenience it would be nice to have this in pandas.

```
series.str.decode('utf-8')
```

is nicer than for example

```
series = Series(['a','b','ä'])
Series(np.char.decode(series.values.astype('|S255'), 'utf-8'))
```

I find myself having to decode and encode a lot depending on what other libraries accept/return. 
