Integration testing of DynamoDB mapper

Here’s the beginnings of an integration test suite.

AWS recently [announced](http://aws.typepad.com/aws/2013/09/dynamodb-local-for-desktop-development.html) a client-side database and server called [DynamoDB Local](http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Tools.html).

This pull request provides bash scripts to fetch, locally deploy, start, and stop this tool. And it is integrated into SBT. If the SBT settings key `localMode` is set to true (it is by default) then SBT will spin up DynamoDB Local, run the integration test suite locally, and shut everything down after the tests have run.

Some initial code has been written to test some of the read methods of `AmazonDynamoDBScalaMapper`.

Note that AWS credentials for scratch code and integration tests are now being sourced from a properties file.