handlers: use SimpleReduceHandler for collections.Counter

We were not handling collections.Counter objects.  Register the
SimpleReduceHandler for collections.Counter objects so that they can be
properly serialized.

Add unit tests to cover the keys={True,False} use cases.

Closes #62

Reported-by: Alex Raichev <alex@raichev.net>
Signed-off-by: David Aguilar <davvid@gmail.com>