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.
Correct me if I am wrong, but if it is to be backwards compatible, it should have a default implementation returning true?

The only easy way I see at the moment to have a default implementation, is to make BuiltinDatastoreExportConfiguration into an abstract class (it is an interface now) but that may break code that depends on it being an interface; for instance a class that implements BuiltinDatastoreExportConfiguration and extends some other class.

Another, more complicated way, would be to make an abstract class that implements BuiltinDatastoreExportConfiguration, which adds the runAsService method and has a default implementation. Then BuiltinDatastoreToBigqueryCronTask would have to test whether the configuration is an instance of this new abstract class. But this will make the code much harder to follow.