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
I'll probably move the createDefaultBucket calls into the test superclass. The whole point of it is to reduce code repetition, after all :)