Django-s3sign

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* Use default max file size of 2gb.
* Update default expiration time to 8 hours.
* Introduce configurable region name, via
`AWS_S3_REGION_NAME`. Defaults to us-east-1.

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* Use presigned POST request instead of PUT.
* Add a configurable policy limit (default 4mb) for uploads.

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* Move s3 signing functions out of the view so they can be imported
and used separately.
* Add bmp image support.

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* This library now requires boto3 and botocore to be installed.
* The `amz_headers` view option has been replaced by the `acl`
option. This is 'public-read' by default. Set it to None or
'private' for non-public assets.
* Use boto3 for signing the PUT request.
* Use AWS Signature version 4 for signing URLs. This avoids the needs
for quoting which has led to encoding bugs.
* Add a ContentType fix on PUT request that fixes a bug where SVG's
can't be uploaded.

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* Fix bug where signatures with '+' in weren't being properly encoded.

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* Add `SignS3View.private` flag to determine whether we're uploading
private resources. If so, return the signed GET url so the media can
be displayed immediately on upload.