Load custom time extensions for Numeric when activesupport isn't available

* Removes activesupport as a hard runtime dependency
* If activesupport is available in the current environment then use that
  for maximum compatibility and to avoid conflicting monkey patches
* If activesupport isn't available, then our version provides all the
  functionality required for specifying cron schedules
stop requiring thread from the stdlib

* it was only there to avoid an active support compatibility issue in
  rails 2.3
Maintain 1.8 compatibility

* Object#singleton_class only became available in ruby 1.9.2, so we need
  this for backwards compatibility
Merge pull request #471 from yob/no-activesupport-redux

Remove runtime dependency on Active Support