#1 Remove SPL dependency

[Paul Chandler]

If SPL is not enabled, xrange should gracefully adjust to the situation and only implement what bare PHP supports. This means stripping the Countable and SeekableIterator interfaces, the two most recent additions. It would be acceptable for an end-user to accept that xrange offers no SPL functionality when SPL is disabled. SPL might be disabled for whatever reason, I dare not ask why. Dependencies == bad.