short description: added mapzen elevation service cleaned up the error handling and logic for elevation services. if an elevation service returns a negative elevation, a very clear warning is shown, and then we fallback to the next service. blacklist for bad elevation services, so if a service returns an invalid value, the service is blacklisted for 1 hour. this is a performance speedup since it doesn't waste time querying the bad service everytime we get elevation. priorities are: mapzen -> mapquest -> google -> random (random elevation between 5 and 50) why this order? mapzen is fast and reliable once you get an api key. no daily limits. mapquest does not require a key and for most users returns a good elevation value. google is last because i think using it for elevation u