plugin datalog

stamp dose not insert unix timestamp

fails readme example with filepatterns and logpatterns enabled:
```
[datalog]
   class_name = DataLog
   class_path = plugins.datalog
#   path = var/log/data
   filepatterns = default:{log}-{year}-{month}-{day}.csv | yearly:{log}-{year}.csv
   logpatterns = csv:{time};{item};{value}\n
#   cycle = 300
```

