Allow region to be passed as argument.

I have a bucket in the US West (Oregon) region. When attempting to upload files to this bucket, I would get the following exception:
```
/home/mloar/devel/belltowncrime/node_modules/s3-cli/node_modules/s3/node_modules/aws-sdk/lib/request.js:34
        throw e;
              ^
PermanentRedirect: The bucket you are attempting to access must be addressed using the specified endpoint. Please send all future requests to this endpoint.
    at Request.extractError (/home/mloar/devel/belltowncrime/node_modules/s3-cli/node_modules/s3/node_modules/aws-sdk/lib/services/s3.js:257:35)
```

This pull request adds region as an argument and passes it to aws-sdk in the config, allowing it to select the correct endpoint.