Jwang limit resource access

Check the authentication header of an HTTP request to see if it's from an admin with the correct password. Limit the "shutdown" REST API to be accessed by admin only. 
Why not use a [`MyriaApiException`](https://github.com/uwescience/myria/blob/master/src/edu/washington/escience/myria/api/MyriaApiException.java#L40) here, which is our wrapper for WebApplicationException that has custom handling?

Also, please provide an `explanation` string, c.f. #636