Replace args '*log.prefix' for '*log.tag', add help message.

Replace args '*log.prefix' for '*log.tag', add help message.

+Do the replacement at code, docs and unit tests.
+Add a help message that's printed whenever there's an error at the configuration arguments.
+Fix a broken unit test.
+Fix the description of 'validation-log.tag'.
+Fix some errors at configuration examples ('examples/config.json') and at the web docs ('usage.html#--configuration-file').