Add multipart upload capability to erlcloud

* Add new put_multipart_object function to erlcloud_s3 module and
create a new erlcloud_s3_multipart module to contain the
multipart-specific functions.
* Add upload_file/5 to strip any path information when determining the
value of the key for storing the file and upload_file/6 to allow the
key to be specified independently of the file name.
* Add functions to list active uploads and the parts of an active
  upload.
* Add functions to convert the XML output from listing all active
uploads or all the parts of an upload to an erlang term.