Refresh Token + Init + Docs Updates

 * Adding `Client#refresh(callback)` for refreshing current token
 * Improving init (breaking change)
    * allows client id/secret to be optional
    * allows setting pre-determined access_token or auth (eg: shared from web app)
 * Updating docs (got a little stale)

https://treetopllc.jira.com/browse/NH-3510