S3transfer

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* enhancment:ProcessPool: Adds user agent suffix.

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* feature:``ProcessPoolDownloader``: Add ``ProcessPoolDownloader`` class to speed up download throughput by using processes instead of threads.

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* bugfix:``RequestPayer``: Plumb ``RequestPayer` argument to the ``CompleteMultipartUpload` operation (`103 <https://github.com/boto/s3transfer/issues/103>`__).

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* enhancement:``max_bandwidth``: Add ability to set maximum bandwidth consumption for streaming of S3 uploads and downloads

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* bugfix:TransferManager: Properly handle unicode exceptions in the context manager. Fixes `85 <https://github.com/boto/boto3/issues/85>`__

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* feature:``TransferManager``: Expose ability to use own executor class for ``TransferManager``