* bumped version to 0.5.0
 * revised data saving & submission process (only save sitemaps, robots.txt & hosts.txt now)
 * revised special scheme handling (registered as site custom hooks)
 * added a demo for data models
 * revised Redis usage (added queue_hostname for new host detection)
 * added DARC_REDIS to toggle waiting Redis connection
 * added redis_command to retry Redis commands
 * retry all Redis commands
 * revised docker integration
 * revised & updated docs