Easy integration in Ruby 2.0

We're upgrading to Ruby 2.0, but need to keep using Syck for YAML as we have
a huge set of Syck-serialized data that Psych doesn't consistently parse the
way we expect. This pull request makes the syck gem a drop-in replacement for
Ruby 1.9.3's swappable YAML:

* A gemspec is included in the repository. I left the Hoe stuff in the Rakefile,
  but it's important that the native code is built when the gem is installed,
  not when the gem is packaged for release (that's what's going on
  [here](https://github.com/tenderlove/syck/issues/1)). Maintaining a gemspec
  by hand is pretty straightforward, and Rubygems takes care of the rest.
* I ported the `YAML::EngineManager` implementation from Ruby 1.9.3. So with
  this gem installed and loaded, you can simply run
  `YAML::ENGINE.yamler = 'syck'` just like in Ruby 1.9, and everything works.