Betty-cropper

- Packaging/install fix

- Improvements to BETTY_CACHE_FLUSHER support:
- `BETTY_CACHE_FLUSHER` can either be set to a callable object or a string import path
- Flusher now passed list of string URLS instead of individual strings. This allows for more efficient callback batching logic.
- Added reference cache flusher `betty.contrib.cacheflush.cachemaster`
- `Image.clear_crops()` now includes animated files (`.../animated/original.{gif,jpg}`)
- Add optional Docker support

- Allow configurable "Image JS" cache time via `settings.BETTY_CACHE_IMAGEJS_SEC`. ImageJS requests are cheap but make up over 50% of current production requests, and only rarely changes on deploys.
- Increase (rarely called) "crop redireect" cache times to 1 hour, a good balance between fewer requests and not overcommitting in case this ever changes.

- Fixes max-width resize regression for cases other than "JPG mode=RGB". Switch to IO buffers requires passing
a `format` value since no longer a filename to auto-detect via extension.

- Management command `change_storage_root` uses older option format for compatibility with Django 1.7

- Added `settings.BETTY_CACHE_CROP_SEC` to allow configurable crop (and animated) cache times. Defaults to original `300` seconds.