Add the S3Bucket class
Remove unnecessary 'require s3file' from upload.rb

s3file.rb is already required by putobject.rb.
Replace bucket_name and region with an S3Bucket

Use an instance of the S3Bucket class to combine the information that
used to be split betwen two parameters to PUTObject's initializer.
Add the AWSKey class
Use an AWSKey instance instead of separate ID, key
Stop passing the date to PUTObject's initializer

PUTObject instances can calculate the current time just as easily within
their own initializers.
Add the 'host' method to the S3Bucket class
Call S3Bucket's 'host' method from PUTObject