PHPCS: code style errors detection and fixes

[Contributing section](https://github.com/composer/composer#contributing) mentions that the code should follow [Symfony coding standards](http://symfony.com/doc/2.0/contributing/code/standards.html).

I added [PHP_CodeSniffer](http://pear.php.net/package/PHP_CodeSniffer/) as a dev requirement and applied fixes to detected errors.

If you don't want `composer.json` changed, I can remove it from [the commit](https://github.com/MacDada/composer/commit/1be33aef293b9afaeadd136b2ec27e5ee9bdd2d7).

There are 2 errors that cannot be fixed simply:

* [CreateProjectCommand->installProject()](https://github.com/composer/composer/blob/master/src/Composer/Command/CreateProjectCommand.php#L132)  should put required arguments first, optional later.
* [JsonConfigSource->array_unshift_ref()](https://github.com/composer/composer/blob/master/src/Composer/Config/JsonConfigSource.php#L140) has no visibility set and is `under_score` style. Because it's `public` by default, chaning that might be a BC break.