v0.12.dev8: Store attachments as intact files (#659)

* attachments are saved as intact files

* reform attachments and filepath

* complete implementation of external storage except for `clean`

* refactor external storage

* complete refactoring of external storage for version 0.12
all tests pass

* rename attribute `basename` to `attachment_name` in external table

* add __repr__ to ExternalMapping

* external files are not copied if stage and store are the same

* make external tables require setting the `delete_external_files` argument

* update CHANGELOG

* fix Python 3.4 compatibility

* fix Python 3.4 and 3.5 compatibility

* dropped support for Python 3.4

* minor changes in error messages

* Update to pathlib in test init.

* Update test_blob_migrate to be compatible for WIN10.

* Fix WIN10 compatibility with KeyboardInterrupt and SystemExit exceptions.

* Fix WIN10 filepath to store as posix and fetch as user's platform.

* Fix relpath for Python3.5.

* Fix copytree for Python3.5.

* Fix typo.

* Fix for Python3.5.

* Update coveralls.

* Update coverall env vars.

* add environment variable DJ_SUPPORT_FILEPATH_MANAGEMENT to enable/disable the filepath datatype

* Update CHANGELOG.md

Co-Authored-By: guzman-raphael <38401847+guzman-raphael@users.noreply.github.com>

* Update CHANGELOG.md

Co-Authored-By: guzman-raphael <38401847+guzman-raphael@users.noreply.github.com>