Complete FTP update script

* Broke initial script into two parts, a DB-specific component and
  the initial script to be run in a chron job

* Started writing tests for the database component. More are needed but
  the DDL queries are currently tested.

* Moved FTP files to a better-named directory, nlm_data_import

* Moved medline_xml test logic from test/__init__.py to nlm_data_import
  nlm_data_tests. These aren't yet incorporated with the download flow;
  the job that runs the parser needs to use them somehow.