OMG a ridiculous commit to overhaul import handler config etc.

- register all import/export handlers via setup.py and config
- use "handler key" lookup for all import/export commands
- fix logic bugs w/ CORE -> Trainwreck importer

rattail_corepos/trainwreck/commands.py

@@ -33,4 +33,4 @@ class ImportCore(commands.ImportSubcommand):
     """
     name = 'import-corepos'
     description = __doc__.strip()
-    handler_spec = 'rattail_corepos.trainwreck.importing.corepos:FromCoreToTrainwreck'
+    handler_key = 'to_trainwreck.from_corepos_db_office_trans.import'