Replace the in-house stubbing by the facilities offered
by pyfakefs for running tests. We don't use `io.open`
anymore, because pyfakefs does not stub it correctly.
We rely on a patched version of pyfakefs, as a few bug
remain in the official repo.
The implementation is designed to be robust to filesystems having integers
or nanosecond stat time.
Added a requirements.txt file for the tests:
`pip install -r tests/requirements.txt` to install the necessary packages
to run the tests.
Fix#13, Fix#14
