Add PSR-0 support.

This breaks BC, yep. In the first commit, I am that asshole:

- Move classes to separate classes from `base_meli.php` and
  `mali.php`.
- The only renamed class was `Meli` itself, to `Meli\Api`.
- TestCase changed to reflect rename.
- Removes trailing spaces.
Add support for Composer.

- Add composer support on make (install, test).
- Support on composer.json to expose on Packagist.