Upgrade notes for SimpleSAMLphp 1.5

* `SimpleSAML_Session::isValid()`

  If your code calls `$session->isValid()` without an argument, you will now have to update it to pass an argument (probably `saml2`).
  The reason for this change is that calling `$session->isValid()` without an argument can easily create a security hole.

* We have introduced a new module for SAML authentication.
  This authentication module supports both SAML 1.1 and SAML 2.0 IdPs.

  We have also added a new authentication framework which should replace the previous redirects to the initSSO-scripts.
  Relating to this change, we have also deprecated the `initSSO`-scripts for SAML 1.1 and SAML 2.0 authentication.
  The old methods will still be supported for a while, but new code should probably use the new code.

  See the [migration guide](simplesamlphp-sp-migration) for more information about this.

* The `request.signing` option has been removed.
  That option was replaced with the `redirect.sign` and `redirect.validate` options, and has been depreceated for one year.

* The `aggregator` module's configuration file has changed name.
  It was changed from `aggregator.php` to `module_aggregator.php`.