Upgrade to Guava 15.0

Release notes:

https://code.google.com/p/guava-libraries/wiki/Release15
Revert "Disable timed temporary signed URLs for AWS-S3"

This reverts commit bc0abbaa073a9178f3aeb0d1541ea5fc50dded35 and
re-enables temp-signed URLs for S3.

Closes JCLOUDS-200
Delete containers after integration tests

Also return newly allocated containers to the pool.
[trivial] Remove unused variables