Allow head requests for S3 buckets

I am simply trying to do a head request to check the existence of an S3 Bucket.

If I were using the aws-sdk, I would use the [aws-sdk's `headObject`](http://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/S3.html#headObject-property) method and do this:

``` js
var AWS = require('aws-sdk');
var s3 = new AWS.S3();

s3.headObject({Bucket: 'someBucket'}, function(err, data) {
if (err) {
console.log(err, err.stack);
} else {
console.log('The bucket exists!');
}
});
```

I see that knox has a `client.head()` method, which seems to need a path to a file, like this:

``` js
client.head('path/to/file').on('response', function (res) {
// ...
});
```

But is there a way to use knox to just grab the HEAD of a bucket?

Something like:

``` js
client.head('bucketName').on('response', function (res) {
// ...
});
```

Thanks!

Start at the existing client.head() entry point, using the AWS SDK headObject example in the issue as the expected behavior. Done means a bucket-only HEAD request is accepted and its response can distinguish whether the bucket exists.

Written by the indexing model from the issue text.