Added Aws::S3::Object#upload_file abstraction.

The `#upload_file` method makes it easy to upload a file from disk
to S3:

    # given a S3::Object
    obj = Aws::S3::Resource.new.bucket('aws-sdk').object('key')

    # upload the file from the given path
    obj.upload_file('path/to/source')

You can pass a String, Pathname, File, or Tempfile to `#upload_file`.
If the file is larger than 15MB, it will be uploaded using the Amazon S3
multipart upload APIs.

    # upload large files in parts
    obj.upload_file('/path/to/very-large-file')