Start work on the csv file importer.

* src/import_csv.cpp: Rename to ...
* src/importers.cpp: ... this. New file, implements .csv parser.

* src/thrift_client.cpp: Reorder header, no functional change.

* src/zipkin_import.h:
  (SendTo...Server): Using namespaces.
  (Importer, CSVImporter): New classes.

* test/import_tests.cpp: New unit test file.
* test/endpoints.csv: test file for the .csv import unit test.