commit config database only after all settings complete
don't put database connection into args

Database connection get passed into individual functions instead of put
into args namespace.
language change: providers "registered" not "configured"

Changed language used to talk about provider registry, rather than
configurations. Hopefully registering sounds more appealing to non-technical
users than configuring.
added entry points: `oai-harvest` and `oai-reg`
docs: updated in-code documentation
docs: updated package description
docs: added examples of use to README