- no need to refer to github; point to the main repositories
on git.openstack.org and remove the requirement for a github
account in order to contribute;
- update few links which have been moved around and fix at least
the associated message of a link;
- link to the website for HTTPie.
Change-Id: Ib310815d346e2364de1a2282f3f1e2d94590c3d2