Refactor Symphony Tasks by extending SimpleHttpOperator

[jermnelson]

The first implementation of the Symphony Tasks returned a SimpleHttpOperator after setting Symphony specific HTTP headers and body depending on the action. This was refactored in #77 to use a PythonOperator because upstream tasks XCOM variables were not available when passing in values to the functions parameters. We're using the requests package to handle HTTP requests within these functions which includes handling HTTP errors.

An alternative would be to extend the existing SimpleHttpOperator with a custom operator and specify the incoming parameters marc_json and token to the NewMARCtoSymphonytemplates.