python 2(.7 at least) support for EnforceOverrides

[tmasek]

duplicate enforce.py file in python 2(.7) syntax: super() with two arguments, and __metaclass__ attribute (instead of class def kwarg)

also modified test_enforce.py to always run (not just for python version 3+)

[tmasek]

hmm, something seems off in my testing. keep getting the error that says a class overrides a method but is missing the @overrides decorator, even though it's there in the code. maybe wait on this PR until I get to the bottom of it..

[tmasek]

sorry, didn't look at this at all during the holidays. looks like in my use case, the failures were all caused by @classmethods; it also looks like that's been fixed (#30), so I think this python 2.7 back port should be good to go

[mkorpela]

Unfortunately not supporting Python 2 anymore