Add persistent connection logger

[evalphobia]

re-implement #3 and refactored testing for persistent connection.

• Add New() function to create hook with persistent fluentd logger
  • it could use same connection
• Change testing to check the message for both New() and NewHook()
• Change hook struct from unimported to imported
  • Allow user to customize fluentd logger

[coveralls]

Coverage increased (+1.0%) to 84.431% when pulling 2663255 on feature/persistent-conn into 9344206 on master.

[coveralls]

Coverage increased (+1.0%) to 84.431% when pulling 2663255 on feature/persistent-conn into 9344206 on master.

[coveralls]

Coverage increased (+4.0%) to 87.425% when pulling 1b4c20a on feature/persistent-conn into 9344206 on master.