Support DynamoDB PITR

[jungseoklee]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

Summary

This patch supports DynamoDB PITR (Point-In-Time Recovery).

Test

~/aws-cdk/packages/@aws-cdk/aws-dynamodb $ npm run test
OK: 33 assertions (365ms)

=============================== Coverage summary ===============================
Statements   : 97.5% ( 78/80 )
Branches     : 91.84% ( 45/49 )
Functions    : 100% ( 18/18 )
Lines        : 97.4% ( 75/77 )
================================================================================
Verifying integ.dynamodb.js against integ.dynamodb.expected.json... OK.

Notes

I updated integ.dynamodb.expected.json through npm run integ command.

[jungseoklee]

Thanks for a quick review!