Merge pull request #470 from yob/remove-internal-activesupport-calls

Remove internal activesupport calls
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