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.
I know that <?php consume following space.
That case does not test if fix method will proceed to next namespace or just return content untouched. That is why I asked for case where `there is no whitespace token before _second_ namespace`