Created authentication service which can be used by the theme

@hippee-lee @tsanko I have created the Authentication Service. Right now, it has the basic functionality to authenticate the user, and I've tested it against my local storefront to make sure that it works Cross Domain. The changes that I made on the server side have not yet been deployed out to samplestore.
Another option here is to return {publicFnName: publicFn, . . . } and keep the return object looking clean but that is just opinion/persnoal preference. 

One thought on the login function: Why not create and return a promise here? Then caller can vnAuthService.login(creeds).then( // implement theme or app specific follow up things when properly authed) or catch the error and do other stuff.