Documentation changes, class name changes, runAsService()

WARNING: These changes are incompatible with the current version and will break existing code!

These changes make rename some classses, to make them consistent with the documentation: Renamed LogsFieldExporter to BigqueryFieldsExporter and LogsFieldExporterSet to BigqueryFieldsExporterSet.

The readme file is expanded, to include some things that I needed to do to get things working.

The parameter run_as_a_service was always true, but I did not have sufficient permissions and introduced the possibility to make this false.
this is optional and only if they want to use datastore export