Currently, in-place operations on 'collections.Counter' with unsupported types raises an 'AttributeError'.

Example:
>>> import collections
>>> counter = collections.Counter()
>>> counter += 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/collections/__init__.py", line 709, in __iadd__
    for elem, count in other.items():
AttributeError: 'int' object has no attribute 'items'

Instead, it should return 'NotImplemented' if 'other' is not a 'collections.Counter'