Add documentation for image url
Support both v1 & v2 glance API

Fix #168

Endpoint /images exists on both v1 and v2 API
The attribute 'visibility' is used to detect
if the call has been made on v1 or v2

In case of v2 we have all the needed information,
but in case of v1 we don't and we have to call
/images/detail to get full details
Merge pull request #181 from ggiamarchi/issue#168

Issue #168 Support both v1 & v2 glance API