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.
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Do you want the code to automatically add a newline before the namespace?