Support for gzipped pipeflow

Using the gulp-gzipped plugin, pipe contents can be gzipped before being uploaded to s3.  To have s3 serve the gzipped content correctly headers must be set.

Purpose:
1) Added gzipped logic
2) Documentation

Future proposal:
It would be good to have the ability to pipe into s3 plugin directly after the gzip step so the contents do not need to be written to disk.  This might be a limitation on the knox client though if file contents cannot be piped.