Implement apiclient library

This library can be used in novaclient, keystoneclient,
glanceclient, and other client projects. The library
contains common code and uses python-requests for
HTTP communication.

Features:
* reissue authentication request for expired tokens;
* pluggable authentication;
* store authentication information in a keyring;
* rich exceptions hierarchy;
* utils for building CLI tools.

Partially implements: blueprint common-client-library

Change-Id: Ic8b466a57554018092c31c6d6b3ea62f181d7cef