v0.5.1

• Include process definition in processes, tasks and payloads to aid debugging.
• Increased test coverage for process and task specs.
• Removed statsd instrumentation.
• Removed unused Taskinator::Visitor::XmlVisitor implementation.
• Various refactorings and clean ups.
• Bug fixes for process class when used as a sub-process
• Add handling for unknown types when deserializing old processes
• Raises UnknownTypeError when trying to invoke processes or Jobs of unknown types