Add a sonos proxy to separate sonos call logic from skill logic

[pheintzelman]

Calls to sonos should be made through a proxy. This helps protect the skill from any design changes to node-sonos-http-api. It also simplifies the logic in the skill.

e.g. sonosProxy.setVolume('Living Room', 10);

instead of intents all building urls

[pheintzelman]

I am happy to do this work but would like comments from others if they have them.

Work in progress can be seen here: https://github.com/pheintzelman/echo-sonos/blob/master/lambda/src/sonosProxy/sonosProxyFactory.js