docs: fixes to docs

Signed-off-by: Ivan Masár <helix84@centrum.sk>
docs: fix installation

* Uses the pip1.6 ready way of installation, no dependency links.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
installation: cerberus package upgrade

* Upgrades Cerberus package in order to support some new features.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
restful: fix for testing accesstoken

* Renames class RestValidator to RESTValidator.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>