About debug and log

[lwhile]

1. About debug:
   I think fmt.PrintX in code can be replaced by logger.

ethereum-watcher/watcher.go

Line 478 in 2c339ec

fmt.Println("removing tail block:", watcher.SyncedBlocks.Back())

2. About log:
   Allow package users to choose whether to use the logger. For example, provide a debug mode.

Also, I can provide a PR if needed.