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.
But static methods should go before instance methods, cf. `CONTRIBUTING.md`.

Perhaps your comment should be added to `CONTRIBUTING.md` though, as the file makes no mention of putting public elements before protected/private ones.