Support for signature version 4

It seems that knox doesn't support signature v4 (http://docs.aws.amazon.com/general/latest/gr/signature-version-4.html). On Frankfurt data center (eu-central-1), it is required, and I can't get knox to work with it. Trying to get a list of files in bucket results following error:

'The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256.'

Here is the sample code:

```
var knox = require('knox');

var client = knox.createClient({
key: '*****'
, secret: '*****'
, bucket: 'my-bucket'
, region: 'eu-central-1'
});

client.list({}, function(err, data){
console.log(err, data);
});
```

Start with the sample client.list call and the AWS Signature Version 4 documentation linked in the issue. Trace how Knox authorizes requests for the eu-central-1 region, then verify completion by making the bucket listing succeed with AWS4-HMAC-SHA256.

Written by the indexing model from the issue text.