Implementation of CAS (Single Sign-On) authentication for DSpace

The first patch was formerly implemented by Kaboré Wendin-Malegdé Patrick at JIRA (https://jira.duraspace.org/browse/DS-1028).
This first version was done for DSpace 1.7.2, afterwards user kgunn at JIRA did some arrangements to make it work with DSpace 1.8.2 and CAS 3.1.x
and now I've been able to make a 'remix' of both solutions to make it work with DSpace 3.1 and CAS 3.5.x.

Hopefully, this patch will be officially implemented in subsequent DSpace versions as an additional authentication method.