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. 
Should really use [`File.separatorChar`](https://docs.oracle.com/javase/7/docs/api/java/io/File.html#separatorChar) here, though of course if anyone uses Myria on Windows there will probably be other issues.