Refactor into ES6 class, add more tests and update docs

[developius]

Signed-off-by: Finnian Anderson get@finnian.io

This converts the openfaas package into an ES6 class, adds more tests and updates the documentation accordingly. It also removes the bluebird dependency in favour of native promises.

The reason for the refactor was to add support for basic auth and other configuration options as outlined in austinfrey/node-openfaas#5. The options are now proxied through to got and by extension http.request, allowing much finer-grained configuration with sane defaults.

[alexellis]

This needs rebasing and cannot be merged right now.

There may have been valid reasons for using bluebird such as compatibility with the serverless framework. Please look into this.