Version 0.13 -- better 1.9.3 and 2.0.0 compatibility

* Fix output of eight queens example under Ruby 1.9.

* Clean up how we detect what Ruby version we are running under (used to
  detect if we have to get callcc from a stdlib (1.9 and above) or if it's
  a builtin (1.8).

* Note support for Ruby 2.0.0.