Fix pickling of partials when there are no kwargs

Fixes errors in the form:
```pycon
Python 3.4.1 (default, May 19 2014, 17:23:49) 
[GCC 4.9.0 20140507 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import functools, dill
>>> def f(a, b, c): pass
... 
>>> g = functools.partial(f, 1, 2)
>>> dill.copy(g)
(<class 'TypeError'>, TypeError('type object argument after ** must be a mapping, not NoneType',), <traceback object at 0x7f9715a66088>)
<function _create_ftype at 0x7f97120272f0> (<class 'functools.partial'>, <function f at 0x7f9715a621e0>, (1, 2), None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/matthew/GitHub/dill/dill/dill.py", line 127, in copy
    return loads(dumps(obj, *args, **kwds))
  File "/home/matthew/GitHub/dill/dill/dill.py", line 160, in loads
    return load(file)
  File "/home/matthew/GitHub/dill/dill/dill.py", line 150, in load
    obj = pik.load()
  File "/usr/lib/python3.4/pickle.py", line 1036, in load
    dispatch[key[0]](self)
  File "/usr/lib/python3.4/pickle.py", line 1382, in load_reduce
    value = func(*args)
  File "/home/matthew/GitHub/dill/dill/dill.py", line 313, in _create_ftype
    return ftypeobj(func, *args, **kwds)
TypeError: type object argument after ** must be a mapping, not NoneType
>>>
```
This occurs when there are no kwargs. Tested with python 2.7 and 3.4.