Create DynamoDB store

[zpatrick]

We currently have in-memory implementations for our database object. Since this service will be stateless, we need to use a real database for persistence. I recommend we use this DynamoDB library.

We will need the following:

• [x] Add new configuration variables:
  • aws-region (envvar = SB_AWS_REGION) (default="us-west-2")
  • aws-access-key (envvar = SB_AWS_ACCESS_KEY)
  • aws-secret-key (envvar = SB_AWS_SECRET_KEY)
  • dynamodb-table (envvar = SB_DYNAMODB_TABLE)
• [x] Create a DynamoDBStore object that satisfies the db.Store interface