Version 0.5.0

• Performance boost! Removes redundant exists call from wp_cache_get(), which easily halves the number of Redis calls.
• Uses add_action() and $wpdb in a safer manner for compatibility with Batcache, which loads the object cache before aforementioned APIs are available.
• For debugging purposes, tracks number of calls to Redis, and includes breakdown of call types.
• Adds a slew of more explicit test coverage against existing features.
• For consistency with the actual Redis call, calls del instead of delete.
• Bug fix: If a group isn't persistent, don't ever make an exists call against Redis.