Fix problem with pickling files

I was trying to pickle files earlier and I think I've found a bug:
```pytb
Traceback (most recent call last):
  File "example.py", line 2, in <module>
    dill.dumps(open(__file__))
  File "/home/matthew/Programming/C++/Python/eggs/dill/dill.py", line 135, in dumps
    dump(obj, file, protocol, byref)
  File "/home/matthew/Programming/C++/Python/eggs/dill/dill.py", line 128, in dump
    pik.dump(obj)
  File "/usr/lib/python3.3/pickle.py", line 235, in dump
    self.save(obj)
  File "/usr/lib/python3.3/pickle.py", line 317, in save
    rv = reduce(self.proto)
TypeError: cannot serialize '_io.TextIOWrapper' object
```
It can be fixed by adding type for the file type definitions.