Added set-endpoint! and updated AWS-SDK

Hi, i had problem with using different s3-backends. Therefore i added new function set-endpoint!, but it also requires newest version SDK. 

There's also little usage example in [.lein-history file](https://github.com/timgluz/clj-aws-s3/blob/master/.lein-repl-history)

Here is also link to [my issue]( https://github.com/weavejester/clj-aws-s3/issues/18)