Support for KMS server-side encryption?

[zezke]

I've made some changes to support KMS encryption. You can just provide a kmsKeyId and it'll set everything up.

Is this interesting enough to warrant a PR?

[kileynaromw]

Hi zezke! Your request appears to be a bit different at first glance, but you may be interested in the PR I opened back on May 20th:

#41

Having this PR merged would allow me as a caller to say something like:

backup = new DynamoBackup({
    ...,
    ServerSideEncryption = 'AES256'
});

It looks like your request KMS encryption is a bit different because my ServerSideEncryption change above uses a default key on the server, but I thought I'd mention it.

[dylanlingelbach]

@zezke yes, this warrants a PR. I lost my write permissions to this repo so trying to figure out if I need to move it. It may take me a while to merge a PR.