FIX support for Ruby 1.9 broken by 4c192be

In Ruby 1.9, Enumerator.new does not take an optional 'size' parameter.
To make the code compatible with Ruby 1.9 (again), a new Yt::Iterator
class is added that inherits from Enumerator but is compatible with Ruby 1.9.
Merge branch 'backport-enumerator-to-ruby19'