always use the new Composer-5.2 autoloader

--HG--
extra : rebase_source : ee13a6f55d244cb92569a555a4d680ee0d50fdf8
removed sly_ClassLoader, as a better implementation is provided for us already
removed counting how many classes were loaded, as this number is now totally misleading
removed PHP <5.1.2 compat
make the autoloader more available
this is done by Composer already