Add dependency on collections.Counter implementation for Python 2.6

As per my suggestion in PR jamescasbon#131, defaultdict(int) was changed
to Counter(). However, I didn't realize it was only added in Python 2.7
and we target Python 2.6.

This approach follows what we already did for collections.OrderedDict.
Merge pull request #133 from martijnvermaat/counter-python2.6

Add dependency on collections.Counter implementation for Python 2.6