Various revisions

- revised accession_settings.txt and initialize_accession_settings()
- revised traverse_bag_contents() to prevent files with no given
extension from being added to the file_types list. Then, revised how the
extensions list was setup in accession_bag()
- created function convert_size_to_string() and this process from
accession_bag() to traverse_bag_contents()
- moved location of traverse_bag_contents() call in accession_bag()
under self.create_import_file. Doesn't need to be called unless
self.create_import_file is True
- added for loop to create_bag_structure() to reuse code
- considered using http://filext.com/ to check whether or not a file
extension exists, but... there are SO many extensions. In particular,
looking to eliminate the false number extensions DA finds, but there's
even '.000' and '.3'. Using the database would eliminate -some-
extensions, but not enough to make a complete implementation viable (for
now)