Adds MappingsConstants class, moving constant in salesforce_mapping.module

to testable constructs.

Adds getter methods getDrupalEntityType and getDrupalBundle to
SalesforceMapping class to mirror getSalesforceObjectType
Fixes referencing errors in PullBase for testability
- Interfaces over classes
- Constant classes over global constable in *.module files
- Removal of t() use and using the implicit calls in the Drupal logger
- Protected methods over private methods
Adds PullBaseTest class (still needs work)
Fixes use list in QueueHandlerTest class
Add TestPullBase class to override necessary wrapper functions for PullBaseTest
Fixes use list in TestQueueHandler