Adding a mixin class to add assertions missing in Python 2.6.
Adding a mixin class to add assertions missing in Python 2.6.

Adds an implementation of assertDictEqual and assertDictContainsSubset.  These
were not added until Python 2.7.

tkanemoto/kombu,Elastica/kombu,jindongh/kombu,urbn/kombu,daevaorn/kombu,iris-edu-int/kombu,ZoranPavlovic/kombu,jindongh/kombu,mverrilli/kombu,numb3r3/kombu,bmbouter/kombu,cce/kombu,iris-edu-int/kombu,bmbouter/kombu,Elastica/kombu,numb3r3/kombu,cce/kombu,tkanemoto/kombu,mverrilli/kombu,ZoranPavlovic/kombu,daevaorn/kombu,celery/kombu