Modify dynamodb-local script to support persistence

[stavros-wb]

Hello,

This PR changes the /test/bin/dynamodb-local script to:
• Don't download the latest dynamodb executable every time
• Optionally pass in a data dir for dynamodb

This provides an easy way to start the local dynamodb (for dev and testing). A dependent project can easily wrap this and use it for starting a local DB

e.g.

"scripts" : {
  "dynamodb:local": "./node_modules/dynamodb/test/bin/dynamodb-local ./.data"
}