S3Proxy 4.0.0

• Answer binary/octet-stream for an object stored without a Content-Type, #179
• Build Docker image on a custom JRE, #401
• Copy multipart parts server-side when possible, #76
• Evaluate the policy a form POST is signed against, #73
• Join multipart parts with copy_file_range on the filesystem backends, #471
• Remove Apache jclouds and the Atmos and B2 storage backends
• Perform conditional PutObject natively where a backend supports them and refuse where it cannot, #627, #894
• Reuse Azure client across multi-part uploads to avoid reauthentication, #1104
• Report the Azure ETag under a composite suffix, #335, #555, #650, #724
• Return and persist flexible checksums, #656
• Support POST upload policies, #73

Thanks @aldrinjenson and @toindev for opening pull requests to improve S3Proxy!