added "all" as reserved provider name
added some logging to harvest module
included metadataPrefix as extension in stored filename

e.g. identifier.oai_dc.xml
fixup logging add commit
docs: improved help message for `--database` option
parser `--from` and `--until` args to datetimes
have OAI-PMH client check timestamp granularity support
don't locally alias args.metadataPrefix
default for `--dir` becomes None

Enables priority selection of correct value for stored providers:
argument > configured > default
added argument for location of persistent config database
docs: updated help for positional `provider` argument
enabled harvesting from named configured provider