implement the proposed snappy framing format

see:
  http://code.google.com/p/snappy/source/browse/trunk/framing_format.txt
      ?spec=svn68&r=55
make the stream compressor automatically choose compression

making the user choose between adding compressed and uncompressed
chunks themselves is a little lame. add some automatic defaults.
add commandline interface
fix suggested stream to stream blocksize
updating README to reflect cmdline usage
upgrade snappy framing format

the framing format changed:
https://code.google.com/p/snappy/issues/detail?id=34
pack/unpack chunk identifiers more efficiently, fix py3
don't mark identifier chunks as skippable
add test for decompression buffering
whoops, forgot to update some docstrings