refactor/iterative retry and abstract poll (#1) * follow Google C++ Style Guide * abstract Poll() * change recursion -> iteration * Update README.md - change exe name - use SRO insteado f dot operator * update HandleHttpResponse fn name * add better err msgs * init Light member variables with default values * add Light::ConvertBrightness * update docs * rm unnecessary empty checks * restructure FetchSingleLightDetails error handling