Option to change the endpoint of S3 deployments

I've added an option `s3_endpoint` to the S3 deployer to enable deployments to other S3 regions.
An example config to deploy to the Ireland (eu-west) region:
```
production:
  deploy_strategy: s3
  aws_key: "**************"
  aws_secret: "*******************"
  deploy_bucket: "www.example.com"
  s3_endpoint: "s3-eu-west-1.amazonaws.com"
```