Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

We can't use an authoritive classmap in the current state as it would
kill app loading. However we can use a proper classmap for the normal
core stuff.

This introduces the defacto standard PSR-4 autoloader from composer into
core. This will allow proper PSR-4 naming of our classes.

Since our original autoloader is still available we can slowly switch
over classes to PSR-4.