Add DynamoDB Handler

This pull request closes #259 (DynamoDB Handler).
As I said in the issue, the only work that needed to be done was to convert the tests to use PHPUnit mocks.

I've also renamed the `FlatStructureFormatter` to `ScalarFormatter` and removed any unnecessary DynamoDB specific behaviour from it (allowing `array(1,2,3)` to remain as a set).

These changes also introduce the [`aws/aws-sdk-php`](https://github.com/aws/aws-sdk-php) to dev dependencies.