Django-optimized-image

This release includes the following changes:
- update the image's `size` attribute after optimizing the image (which allows images to be stored in Google Cloud) (3 )
- increases Django version to latest bugfix release (for Django 1.11)

This release includes the following changes:
- add an optional `OPTIMIZED_IMAGE_IGNORE_EXTENSIONS` setting for image extensions that should avoid optimization project-wide (1)
- increase testing to include python3.7 and Django2.1 (2)

This release includes the following changes:

- remove configuration with S3, and allow using a project's default storage
- added the option to use `Pillow` for optimization
- `OPTIMIZED_IMAGE_METHOD` (now a required setting) setting determines whether to use `"tinypng"`([TinyPNG](https://tinypng.com/)) or `"pillow"`([Pillow](https://pillow.readthedocs.io/)) for optimization