implement the proposed snappy framing format

see:
  http://code.google.com/p/snappy/source/browse/trunk/framing_format.txt?spec=svn68&r=71

7f304a6 implements the framing format in the following manner:
* first, it makes the root snappy module a python module instead of a compiled module, and renames the compiled module to _snappy. this enables a bunch of easy python language modifications and extensions
* second, it exports some c code for the required crc format in the snappy framing format (crc-32c). this turned out not to already be in the python standard library, unfortunately, but the code was generated by the excellent pycrc (http://www.tty1.net/pycrc/)
* third, it implements the actual snappy stream format as a pair of stream compressor/decompressor objects in straight python that match the zlib compressobj interface.
* last, it merges the python 3 and python 2 tests to work under both python 3 and python 2

the other changes are luckily less monolithic.

this pull request should effectively close https://code.google.com/p/snappy/issues/detail?id=34