Making the AmazonS3Client instance a member field to be reused throught the lifetime of the instance.
The problem creating a new instance of AmazonS3Client for every action on the S3FileSystem instance
is that each time you need to establish a new SSL session to S3 with every new connection,
and we found this to be very determental to performance. Thus, keeping the instance cached elimates the
SSL (re-)negotation, and uploads can proceed faster.
Merge pull request #182 from jasobrown/1.1

Making the AmazonS3Client instance a member field to be reused. Changes look good hence merging.