feat: add transcode to compression

[william-silversmith]

cloudfiles/compression.py

def transcode(
  files, encoding, level=None, 
  progress=False, in_place=False
):
  """
  Given the output of get, which may include raw files, 
  transcode the compresson scheme into the one specified by 
  encoding. If the files are raw and the schemes match, 
  the decompression-compression cycle will be skipped.

  level: input of compression level e.g. 6 for gzip
  progress: show a progress bar
  in_place: modify the files in-place to save memory

  Yields: {
    'path': ,
    'content': ,
    'compress': ,
    'raw': ,
  }