Structure refactoring

• Renamed "redis.py" checker to "redis_status.py".
• Renamed "memcached.py" checker to "memcached_status.py".
• Added heartbeat.checkers.host to the default checkers

* The "redis_status" and "memcached_status" refactoring will impact your HEARTBEAT's checkers configuration.

Current valid checker names(for your settings.py - HEARTBEAT['checkers']):

• heartbeat.checkers.build
• heartbeat.checkers.databses
• heartbeat.checkers.debug_mode
• heartbeat.checkers.distribution_list
• heartbeat.checkers.host
• heartbeat.checkers.memcached_status
• heartbeat.checkers.python
• heartbeat.checkers.redis_status