Give S3Bucket the ability to send PUT requests

Copy most of the code from the PUTObject class. I'm still trying to find
an intuitive and easy-to-use interface for the Amazon S3 REST API, and
this feels like a step in the right direction. A separate PUTObject
class seems unnecessary and a bit unwieldy to me at the moment.
Change S3Bucket's send_put_request to send_request

Make send_put_request more generic by renaming it send_request and
giving it an HTTP verb as a parameter.
Make debugging messages in send_request generic
Add HTTParty to the Gemfile
Replace Net::HTTP with HTTParty in S3Bucket
Add the contains? method to S3Bucket
Move auth header calculations to the AWSKey class
Get rid of 'require nokogiri' in upload.rb
Require s3file, aws_key, and utility in S3Bucket
Remove "require_relative 'utility'" from upload.rb
Require 's3bucket' instead of 'putobject'

upload.rb no longer uses the PUTObject class.
Delete putobject.rb

The PUTObject class is now redundant because all of its functionality
has already been moved to S3Bucket and AWSKey.