[WIP] Imrpovements

* Change namespaces 
* Fix composer.json
* Fix autoloading.
* Fix readme


@crevillo You can keep it open and I'll continue pushing changes to it. Or you can merge it in this case I'll open a new PR when I have something. 

root namespace is a vendor name but it is not in the payum yet. So it may confuse people as they may think that the lib comes from payum