Fix isinstance verification in tests

The FakeDateMeta metaclass addition made some assertions "always valid":
the issubclass(obj, FakeDatetime) returns true both for fake and
original datetime objects.

To fix it a new set of helper functions was added. They test for exact
obj.__class__ match instead.
Make sure datetime.astimezone returns the fake object
Update tox.ini

- Support for python 3.4
- make command added to whitelist to suppress the tox warning
Merge pull request #54 from imankulov/fix_astimezone

Fix astimezone