Fixes for the remainder of lib/SimpleSAML

With a dynamic configuration, it is not desirable to load from literal files
By storing a configuration object against a filepath, subsequent calls to
getConfig work transparently, bypassing the disk load step.

Fixes https://github.com/simplesamlphp/simplesamlphp/issues/818

This reverts commit 0917046c.

To convert existing tests to use the base class use

 perl -p -i -e 's/PHPUnit_Framework_TestCase/SimpleSAML\\Test\\Utils\\ClearStateTestCase/g' `grep -rl PHPUnit_Framework_TestCase tests | grep -v ClearStateTestCase.php`

A PHPUnit listener unsets SSP environmental variables and attempts
to restore globals.

When we try to load a configuration file and there's a syntax error, PHP 7 will throw the new ParseError. We can take advantage of that to capture it and give the user a meaningful message back.

This resolves #483.

It is set later inside one of the branches of the if, but the other branches abort the execution of the method, so there's no need to define it outside the if.

Now we are finally using the 2.x branch of the SAML2 library, which was also migrated to use namespaces. Even though the library provides an autoloader that allows loading the classes with the old names using class aliasing, we need to do the migration in one commit (at least for most part of it). This is due to the way SimpleSAMLphp checks data types, using inheritance to check objects agains abstract or more general classes. Even though class aliasing works, there's no way to replicate those relationships, and type checks that use the old class names will fail because the aliases are virtually new classes that don't inherit from others.

Due to recent changes to fix the way we were building URLs (mixing what the 'baseurlpath' configuration option and the current URL, see #396), we introduced another bug by assuming file paths will always use slashes ('/'), which obviously is not true in Windows machines. This commit fixes SimpleSAML_Configuration::getBaseDir() and SimpleSAML\Utils\HTTP::getSelfURL() to take that into account.

This closes #414.

Make sure the new SimpleSAML_Configuration::getBasePath() complies with its phpdoc, returning always at least a slash. Fix also the regex that's supposed to catch paths, as it still wants them to end with a slash.

Add a new SimpleSAML_Configuration::getBasePath() method as described in #364, deprecate getBaseURL() and make sure it generates a warning (and only one) when used.

Be graceful with the 'baseurlpath' configuration option. We should not fail when the trailing slash is missing, just add it.

Start using the new configuration exceptions, handling error situations that before led to blank pages or even worse.

Make it possible to pre-load a configuration array into SimpleSAML_Configuration for subsequent calls to getInstance() to use it. This opens up for unit-testing code that calls SimpleSAML_Configuration::getInstance().

Bugfix: if an optional configuration is missing, we shouldn't register it to reuse it later (i.e. loading a config file as optional first and as mandatory later would make the second call to work -while it shouldn't-, since an empty config was loaded during the first call).