Refactoring

- Remove late static binding (to keep compatibility with PHP 5.2)
- Adding convenience factory method for IP and IPBlock
- Adding isPrivate() and isPublic() to IPv4 and IPv6
Adding caching to getMask() and getDelta()
IPBlock implements Iterator, Countable and ArrayAccess
Refactoring to use GMP for IPv4 and iterators

- Using GMP everywhere makes the codebase smaller and easier to maintain.
- IPBlockIterator can now interate over the entire IPv6 range of addresses
  (not sure you want to do it though, it might take a while)
- Also adding plus() and minus() for blocks
- PHP 5.2 compatibility (removed all late static bindings)