amended s3 plugin to support 'dest' parameter

to allow uploading to custom location.

test.yml follows (though my debug step is incorrect or I don't know how to invoke it...):
````
---

- hosts: 127.0.0.1
  connection: local
  vars:
    projectZipName: build.tgz
    projEnv: dev
    bucketName: my_builds

  tasks:
    - name: Uploading artifact to s3
      action: s3 bucket={{bucketName}} path=build.tgz dest=custom/{{projEnv}}/{{projectZipName}} state=present overwrite=yes expiry=900
      register: downloadUrl
    - name: "{{downloadUrl}} created"
      action: debug msg="{{downloadUrl}} created"
````