multipart object support

 * support for multipart uploading and downloading. This raises the maximum
   supported file size to the S3 maximum of 5 TB. It also allows this module
   to be used to download files which were uploaded via multipart.
 * uploadFile does not always emit 'stream' - in the case of a multipart upload
   it emits 'fdSlicer' instead.
 * default maxAsyncS3 increased from 14 to 20
 * added `multipartUploadThreshold`, `multipartUploadSize`

closes #19