[enh/ref] Add the 'event_source' decorator for fetchers

Hackeragenda is a great tool, and it's awesome that anybody could add its organisation. Although, adding an organisation requires to write boilerplate code, and to modify different portions of `fetch_events.py`. Recent pull requests ( #47 for example ) have shown that we need to reduce as much as possible required boilerplate code.

# Improvements
This patch brings some automations to fetchers:
  * the organisation name is directly inferred from the fetcher function name
  * outdated events records are automatically removed from database
  * fetchers are automatically collected (no more need to declare function; then add its name to a list)
  * logging is uniformised