Add importutils and timeutils from oslo-incubator

They will be needed for the common API client.
Add six library to requirements.txt

It will be needed for the common API client.
Add common apiclient code by Alessio Ababilov

This is code that is likely going to become the common API client
library for all OpenStack python-***client projects.

It was prepared by Alessio Ababilov. It didn't pass the review initially
but hopefully it will make it into Oslo-incubator in the future.

https://review.openstack.org/#/c/27729/
https://blueprints.launchpad.net/oslo/+spec/common-client-library

The code also contains tests/apiclient/utils.py, which is taken from
oslo-incubator tests/utils.py. There are some parts commented out,
because it seems we don't need those and they would introduce additional
(test) dependencies for tuskarclient.
Merge branch 'prq_apiclient' into apiclient