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>
tags: fix for editor in search results

* Fixes broken javascript editor.  (addresses #1792)

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>
linkbacks: fix external url creation

* Fixes the order of keywords arguments in `url_for` that breaks if one
  tries to generate external link.  (closes #1707)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
travis: removal of Python 2.6

* Removing python 2.6 from the test matrix as it times out.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
linkbacks: fix external url creation

* Amends 4755a50867bd34a8debb4b87d952e72b931eefbc.  (addresses #1707)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
docs: typos and fixes

Signed-off-by: Ivan Masár <helix84@centrum.sk>
tags: REST API and testing

* Creates API for tags, in order to have a reusable API
  both for restful API and views probably.

* Refactors tags REST API in order to use the new tag API.

* The new tags API is not yet tested.

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