Update to this plugin

Hi Doug,

First of all thanks a lot for this plugin. I actually used it a lot both at home and at work. I've made some changes to it and hopefully they can be incorporated in the master branch. The main changes are:

- Switch from jets3 to amazonaws library since it's officially supported
- Fix concurrency problem when multiple jobs are uploading the artifacts at the same time.
- Changed the architecture to the new Jenkins standard and update/get rid of deprecated ones.
- Added unit test (so far only 1 but I'm working on it).

Let me know what you thinks. I'm looking forward to your feedback. Hopefully if u don't mind this can be incorporate into the Jenkins main plugin repository.

Thanks a lot,

Long Ho