Added composer to the module to download external libraries.

First draft to use composer to download external libraries.

Currently there is not a good/official way to include external lib with composer

Problems found : 
- This download libraries in a vendor dir inside the Commerce module and doesn't use Drupal vendor dir. This could cause duplicates libraries and conflicts for Drupal. We should have a composer manager to handle that.
- The "auto-install" is run using Symfony process but this seems to be the simple solution right now