Conditionally import weakreset for Python versions < 2.7
Moved weakrefset.py fron extern folder into utils/compat
Removed code that imported weakrefset, was moved to utils/compat
Import utils/compat/weakrefset instead of weakref for backwards compatibility
Use "absolute import" to get utils/compat/weakrefset, add __init__ file
Import weakrefset conditionally
Moved code to conditionally include weakrefset into utils/compat/weakref.py
Merge pull request #20 from bernie-simon/bsimon-python26

Conditionally import weakreset for Python versions < 2.7