Salesforce Term Reference

Adds a new field mapper to the Salesforce suite of modules to allow using a term reference field. Entity references for content are supported natively by the Salesforce modules (https://www.drupal.org/project/salesforce), but term references are not.

This code is largely based on the work of Mike Christianson and he deserves the credit: http://codeaweso.me/2015/04/mapping-salesforce-object-fields-to-drupal-t...