Signed-off-by: Jaime Pérez Crespo <jaime.perez@uninett.no>

This includes instructions on how to use SimpleSAMLphp with the PHP session backend while you are using it too in your application, also with a custom session handler.

It wasn't immediately clear to me where to put the cert and private key when I went through these instructions recently, so I thought I'd propose a change to make the destination explicit.

Closes #1103

* Cleaned up .gitignore a little bit

* Added a .gitignore to modules, ignoring all non-tracked modules.

* Fix internal markdown links in the install docs

* IDP configurable AuthnContextClassRef/AuthnContextComparison

* Add a few namespaces