General: docs, formatting, clean-up

Deprecate some "default_" vars in favor of a new "database_defaults" var, that allows overriding of defaults used by add_database().

Fixes check_connection() so that it doesn't get hung up on itself; adds the ability to pass a custom "reconnect_sleep" duration; renames "allow_bail" to "die_on_disconnect" to avoid a double-negative (false to proceed, but true by default...)

Improves a bunch of docs & updates variable names to modern'ish standards.

See: #64.