Add interval options to dbunloader.py

- Add interval options to dbunloader.py so that it can do more than only
  send the latest records.
Add default value for 'interval' in dbunloader

- Add default value for 'interval' in dbunloader as it is a new option so
  it may not be in the config file.
Merge pull request #45 from apel/iss26-unloader_months

Add interval options to dbunloader.py