Refactoring get_batch_results to be a generator
Refactoring get_batch_results to be a generator

In an effort to provide more flexibility to the api client consumer,
get_batch_results now yields each result record as it is processed.
Because consumers may want not want to process result files serially,
this method is now limited to 1 result_id. A separate method has been
provided to get all of the result ids for a given batch.

heroku/salesforce-bulk,zirmite/salesforce-bulk,wingify/salesforce-bulkipy,chrisguidry/salesforce-bulk,pelucid/salesforce_bulk