Error handling

 - Added more error handling and an error response helper class following the guidelines here: http://docs.aws.amazon.com/AmazonS3/latest/API/ErrorResponses.html
 - Added list buckets and list objects functionality along with corresponding tests
 - Stopped S3Server from creating a default bucket all the time
 - Added .gitignore file
 - Added creation date to buckets
 - Added constants for HTTP methods and HTTP headers
 - Sped up the tests by removing unnecessary calls to Thread.sleep