Change the extension of the documentation files from .txt to .md so that they can processed as markdown and displayed in github.

Update the documentation regarding translation (remove all references to the translation portal, reformat and fix typos).

Update the documentation. Trailing slashes are no longer required for 'baseurlpath'. This resolves #363.

Fixes #379

Add a method to SimpleSAMLphp_SessionHandlerPHP to restore a session existing previously to our own session. This can be used in SimpleSAML_Session to restore the PHP session status previous to calling our API, while also guaranteeing that our session is correctly saved. The documentation has been updated to reflect this and recommend how to deal with conflicting PHP sessions. This closes #244 and resolves #349.

* Remove 404 link to SSP news; this is covered on simplesamlphp.org now.
* Make screenshots link relative, so docs don't trigger insecure
  content warnings.
* Compress png images.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

This reverts commit 09c5a23f.

SHA-1 is still supported but needs to be configured explicitly if you need it.

Revert the new configuration option for the core:PHP authproc. Since we need to serialize authprocs and SP metadata in the state array, and closures are not serializable, it doesn't work. We could create a new module with this, adding a dependency on opis/closure 2.0.* or equivalent, to be able to serialize closures.

Added support for the full SCHAC 1.5.0 schema, including new attribute mappings between the old namespace in TERENA and the new urn:schac namespace.

Deprecate SimpleSAML_Default::initLogin() and move it to SimpleSAML_Auth_Source as a non-static method.

Move SimpleSAML_Auth_Default::handleUnsolicitedAuth() to sspmod_saml_Auth_Source_SP::handleUnsolicitedAuth() and deprecate the former.

Move SimpleSAML_Auth_Default::extractPersistentAuthState() to SimpleSAML_Auth_State::extractPersistentAuthState() and deprecate the former.