Add gzip decoder support #35

- Gzip decoding can be specified via 'decode_gzip', 'extract_gzip', or
  'gunzip'. I added aliases because I'm a nice guy.
- Decoders will always close the IO that they are decoding.
- Overall, improve validation of feeds
Merge pull request #36 from justfalter/add_gzip

Add gzip decoder support #35