Psr0Fixer to tokens

I've rewritten the Psr0Fixer to use tokens instead of raw string, and I've added a leading `<?php` to all test-cases to make `token_get_all` work.

There's one test that fails though: the leading namespace test. Is it truly required behaviour for the fixer to remove leading whitespace? (it isn't mentioned in the PSR-0 standard)

This PR implements the last TODO in #489.
Well, the issue is that the CONTRIBUTING.md file is ambiguous. Because before this statement, it asks to follow the Symfony coding standards, which are asking to put public methods before protected ones, themselves before private ones: http://symfony.com/doc/current/contributing/code/standards.html#structure