Added basic support for OS Keystone

* using `www-authenticate` HTTP header
* only basic/digest and VOMS X.509 authN is supported
Updated VCR cassettes for keystone integration
Partially fixes #50, impl in b351c09

* Updated docs
* See b351c09 for OS Keystone impl
* See dabab25 for updated tests
Minor fixes related to OCCI-OS support

* Minor version bump to 3.1.0.beta.3
* Updated VCR cassettes
Merge pull request #72 from arax/keystone

Added basic support for OS Keystone