Add custom equality function as walk_together argument
Merge pull request #131 from mgymrek/master

Added method to return alt. allele frequencies when there is more than o...
Merge pull request #132 from bow/patch_walk-customfunc

Add custom equality function as walk_together argument
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
Fix unit tests on Python 2.6 and add missing tests to the suite
Fix comparison of _Record objects on Python 3
Adding method to compute heterozygosity for a site