Bucketstore

This release adds:
- Support for alternate s3 endpoints via 25 (thanks fbkopp !)
- Updated docs
- Updated python support list

and removes:
- Support for python 3.5

This release adds some useful new tools for `S3Key`:
- `S3Key.download()` for downloading to a file path or a file-like object
- `S3Key.upload()` for uploading to a key from a file path or a file-like object
- `S3Key.size()` or `len(S3Key)` for getting the size of the object at the given key

as well as adding:
- 100% coverage (minus one line of raising an Exception on other boto Errors)
- more examples in the readme
- python 3.8 testing on Travis-CI

This release updates some of the internals of this library. It adds:
- Type annotations
- `in` operator on `S3Bucket`
- `del` operator on `S3Bucket[key]`
- more documentation

and removes:
- old python version support