Fix/composer autoload

@eps90 i'd like your opinion on this because it seems that you added composer autoloading in the ``bin/mage`` and ``composer.json`` files.

On my environment composer autoloader was never called because the ``__DIR__ . '/../vendor/autoload.php'`` path was leading on no file because ``__DIR__`` is referencing this directory : ``vendor/andres-montanez/magallanes/bin``. So i changed it according to that.

Then, because i wasn't using the internal Mage autoloading anymore but Composer's i encountered the same problem for userland ``Task`` and ``Command`` namespaces because the .mage directory is not correctly referenced. Thus i had to change the PSR-4 autoload directories in ``composer.json``

I guess this has never been spotted because everybody's using Mage with another application that is requiring Composer's autoloading file on its side ?