Support KMS encryption and decryption

Support encrypting and decrypting specific fields with [KMS](https://aws.amazon.com/kms/). Not sure about the syntax though.

Maybe something like this would do

``` js
Table.insert({foo: 'bar'}, {hello: 'world', user: 'baz'}).encrypt(['hello', 'user']).exec();
```

``` js
Table.find({foo: 'bar'}).decrypt(['hello']).exec();
```
