Python3 fixed in cloudpickle.py

Hiya Josh,
This is my bull-in-a-china-shop quick attempt to address some of the python 3 issues in cloudpickle.py. A few of them I'm not sure about (buffer in 2.x vs memoryview in 3.x, types.InstanceType on 2.x vs object in 3.x), but I was able to import/run the module with python 3.4 after the changes. I regrettably don't have spark built at the moment so can't run the tests, but here's hoping this is at least helpful.