Fix key.
Rewrite the look ahead iterator.

It does no longer extend \CachingIterator.
Bye bye.
Re-implement SPL RegexIterator and Recursive~.

There are too many bugs in php-src and HHVM, so we re-implement it from
scratch without extending the existing classes.
Update API documentation.
Fix the specific fileinfo class support.
Fix API documentation.
HasChildren is not very well-defined, remove it.
Remove the recursive tree iterator.
HasChildren and Tree have been removed.
Implement the look behind iterator.
Add tests.