implement JSON handling for Gearman subsystem

- Fact and FactSet classes can now serialize to and from JSON
- Added some test cases for Fact and FactSet classes and JSON
- spawn_worker now accepts one more argument, json boolean switch
- generate_worker can now handle JSON arguments in addition to YAML
- cisco vendor-specific community@vlan hack minor fix to handle default
  community
-gearman-worker.pl can now use --json argument to switch to JSON mode