Add support for gzip compression when making HTTP requests

[mbmccormick]

The Forecast Data API supports HTTP compression. The Dark Sky team recommend using it, as it will make responses much smaller over the wire. To enable it, we simply need to add an Accept-Encoding: gzip header to all requests. The request library supports this through the gzip:true option.

https://darksky.net/dev/docs/response

https://www.npmjs.com/package/request