Avoid issues prior Composer MW 1.22 installation

If a user runs Composer from a local SMW directory, the Composer autoload prior MW 1.22
is run at later point and to ensure LocalSettings remains untouched we register
'SMW\NamespaceManager' to enable enableSemantics() to work its magic.

Class 'SMW\NamespaceManager' not found in ...\includes\GlobalFunctions.php on line 287

Enables to run composer install/update from the SMW root directory for MW 1.19
without the need to change LocalSettings or to install an extra extension.
Merge pull request #52 from SemanticMediaWiki/composer

Avoid issues prior Composer MW 1.22 installation