gzip file reading + test

First of all, thanks for filey-diff and jekyll-s3! 

Added Gzip file support. It may be a little slower since it has to load and unzip each gzipped file to test the md5's, but in testing (with jekyll-s3 on a relatively small site) I don't notice a difference.

Basically it checks if `S3Object[:head][:content_encoding] == 'gzip'` if so it reads the file, unzips it, and computes the original md5 and pulls out the original mtime.

Includes rspec test, passes all existing tests, and works in practice with jekyll-s3: 

I have pretty-much zero past experience with open-source contributions and ruby IO, let me know if there is anything else that needs to be done.