[FEATURE] Make autoloader namespace aware

The autoloader will now recognize classnames with fully
qualified namespaces names. So, it is now possible to
autoload a class called '\Tx\ExtensionName\Foo\Bar' that is
located in 'extPath'/Classes/Foo/Bar.php.

Change-Id: I9b3f130ca201eee94b595c2eee6f64e1c9a63e93
Resolves: #39147
Releases: 6.0
Reviewed-on: http://review.typo3.org/12944
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Daniel Lorenz
Tested-by: Daniel Lorenz
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn