Added Python 3.x classifiers to setup.py.

They were missing :)
Free reference to error callables to prevent memleak when callables
reference an instance.
Merge pull request #29 from yoav-steinberg/memleak_on_callable_errors

Mem leak when using callables that reference an instance
Update changelog and bump version
Merge pull request #28 from thedrow/patch-1

Added Python 3.x classifiers to setup.py.