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've made the requested change in 4b14ca4.
The foreach set-up works though, even though I don't know why. I added a test (see ea344f3) with 100 namespaces (tested with 1,000 as well, but that really slows down the testing process) which worked fine with foreach.