Get a signature key. See:

    http://docs.aws.amazon.com/general/latest/gr/signature-v4-examples.html#signature-v4-examples-python

def _sig_key(key, date_stamp, regionName, serviceName):
    '''
    Get a signature key. See:

    http://docs.aws.amazon.com/general/latest/gr/signature-v4-examples.html#signature-v4-examples-python
    '''
    kDate = _sign(('AWS4' + key).encode('utf-8'), date_stamp)
    if regionName:
        kRegion = _sign(kDate, regionName)
        kService = _sign(kRegion, serviceName)
    else:
        kService = _sign(kDate, serviceName)
    kSigning = _sign(kService, 'aws4_request')
    return kSigning