Added management of groups attribute and modification of SSOlogin

I've proposed a modification to the Shibboleth authentication plugin so that:
* it would be possible to manage an attribute containing user groups (separated by ;)
* the SSOlogin url can be customized to use Login (access directly to one IdP without DS/WAYF) or to use a custom login page (so that the redirect information can be externalized into Apache configuration, for instance).

The code developed does not modify the original behavior but supports different use cases (really relevant to some research activity we're carrying on).

Hoping this modification could be accepted, I've also updated the wikipedia page for this extension at:
https://www.mediawiki.org/wiki/Extension:Shibboleth_Authentication
The modification on this page are as such that the original configuration does not modify the behavior of the extension.

No unit test has been implemented (in effect,  no test is present in the original project) but these modifications have been tested (and are currently used) in a research activity.
More information here:
https://wiki.terena.org/display/gn3pjra3/Grouper+in+a+cross+organisational+context