Allow setting of signatureVersion globally
Use configured endpoint when setting endpoint in S3
Add support for interpolating endpoint configuration parameter

Supported template variables are:

* `{service}` - the service endpoint prefix
* `{region}` - the configured region
* `{scheme}` - the configured scheme ("http" or "https")

For example, the following configuration sets the endpoint to
'SERVICE.REGION.example.com' for all constructed services:

```javascript
AWS.config.endpoint = '{scheme}://{service}.{region}.example.com';
```