Add composer support and use atoum's default configuration and bootstrap files

Now, to launch unit tests, we can do `vendor/bin/atoum` (if we use composer) from the repository's root directory: the configuration files tells atoum where to look for tests.

The `.bootstrap.atoum.php` loads the required autoloader, depending of what is available (composer or not composer).

What's left to do:

* [ ] Update `composer.json` and set the right email address ([https://github.com/jubianchi/Hoathis-Lua/compare/tests#diff-b5d0ee8c97c7abd7e3fa29b9a27d1780R9](https://github.com/jubianchi/Hoathis-Lua/compare/tests#diff-b5d0ee8c97c7abd7e3fa29b9a27d1780R9)
* [ ] Update the path to Hoa. I think it would be better to use a path like `Hoa/Core/Core.php` to let PHP search in its include path ([https://github.com/jubianchi/Hoathis-Lua/compare/tests#diff-83ad6bd4885732fd14b9d8709d592efdR3](https://github.com/jubianchi/Hoathis-Lua/compare/tests#diff-83ad6bd4885732fd14b9d8709d592efdR3))
* [ ] test if everything still works without composer : unfortunately, I don't have Hoa installed here so I can't guarantee I did not break anything