v0.8.0

• Significant refactor of how queues work, which are now global and are stored in the database
• chancy.executors.async_ renamed to chancy.executors.asyncex
• A new plugin, WorkflowPlugin, is now available for simple DAGs.
• Pruner plugin now calls the cleanup() method on the various plugins when it triggers, giving other plugins a chance to cleanup custom tables and this is used by the Workflows plugin.
• Some documentation improvements