Add 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;
* rich exceptions hierarchy;
* utils for building CLI tools.

Partially implements: blueprint common-client-library

Change-Id: I82df865d52f32ce47dde6a09a84d3d917fd77918