[WIP] Pre-gzip html files for sending via nginx

- Fixes #522 

- Uses the `gzip_static` nginx directive
- gzip all of the html files in `build_site.sh`

TBD if this has a material impact on performance, @ycombinator 
I think this line (along with its clones in the other playbooks) should be moved to the `server` section further below so it only affects the developer.rackspace.com server block.