'Begin a multipart upload
http://docs.amazonwebservices.com/AmazonS3/latest/API/index.html?mpUploadInitiate.html'

def initiate_multipart_upload(self):

    if (self.s3.config.upload_id is not None):
        self.upload_id = self.s3.config.upload_id
    elif self.s3.config.put_continue:
        self.upload_id = self.get_unique_upload_id(self.uri)
    else:
        self.upload_id = None
    if (self.upload_id is None):
        request = self.s3.create_request('OBJECT_POST', uri=self.uri, headers=self.headers_baseline, uri_params={'uploads': None})
        response = self.s3.send_request(request)
        data = response['data']
        self.upload_id = getTextFromXml(data, 'UploadId')
    return self.upload_id
