Feature Request: Provide dict object for pd.Series.to_dict()
I frequently use the `.to_dict()` method and was wondering how difficult it would be to implement a `dict_obj` argument to specify what type of dictionary will be used.  For example, if one was interested in preserving the order: 

```
from collections import OrderedDict

pd.Series(list("abcd")).to_dict(dict_obj=OrderedDict)
```