Add AWS Region configuration property

Added AWS Region to the post-processor's configuration so the following is now possible:

```
      {
        "type":     "vagrant-s3",
        
--->   "region":	"us-east-1",  <---  

        "bucket":   "my-s3-bucket",
        "manifest": "vagrant/manifest.json",
        "box_name": "my-cool-project",
        "box_dir":  "vagrant/boxes",
        "version":  "{{ user `version` }}"
      }

```

Valid region values are what AWS uses to identify them:

* us-east-1
* us-gov-west-1
* us-west-1
* us-west-2
* eu-west-1
* eu-central-1
* ap-southeast-1
* ap-southeast-2
* ap-northeast-1
* sa-east-1
* cn-north-1

However, I only tested this on us-east-1....

I've also updated Artifact type to include State method and revised README to reflect above changes.

This PR fixes #3 and also fixes #6