[enh/ref] Add the 'event_source' decorator for fetchers; change fetchers call conventions

This patch brings some automations to fetchers:
  * the organisation name is directly infered 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  * outdated events records are automatically removed from database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
[mod] separate standard lib modules from external deps
[mod] don't use a class variable for SOURCES list
[mod] more pythonic test
[mod] blank lines to make code more lisible
[mod] re-inject logic into the responsible class, only gather functions in the decorator
[mod] pep8
[mod] rename variable to match reality
[mod] be explicit
[mod] remove strange code
[mod] pep8
[mod] remove useless variable
[mod] style
Merge pull request #52 from Psycojoker/event_source_decorator_path

Event source decorator path
Merge pull request #51 from Psycojoker/event_source_decorator

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