Add shebang in bin script

[ymtszw]

Currently, package.json states:

  "bin": {
    "local-dynamo": "./bin/launch_local_dynamo.js"
  }

So npm install local-dynamo in user's project will generates node_modules/.bin/local-dynamo file with contents of bin/launch_local_dynamo.js.

Though bin/launch_local_dynamo.js currently does not have shebang so executing it directly from shells or via npm run will cause syntax error.

This PR adds it so now you can do these from user's project:

$ npm install local-dynamo
$ node_modules/.bin/local-dynamo
Initializing DynamoDB Local with the following configuration:
Port:	4567
InMemory:	true
DbPath:	null
SharedDb:	false
shouldDelayTransientStatuses:	false
CorsParams:	*

# With npm-scripts in package.json like this
# "scripts": {
#   "dynamo": "local-dynamo"
# }
$ npm run dynamo
Initializing DynamoDB Local with the following configuration:
Port:	4567
InMemory:	true
DbPath:	null
SharedDb:	false
shouldDelayTransientStatuses:	false
CorsParams:	*

Also, node binary can handle shebang in passed script, so usage example in README.md should keep working.

$ node bin/launch_local_dynamo.js
Initializing DynamoDB Local with the following configuration:
Port:	4567
InMemory:	true
DbPath:	null
SharedDb:	false
shouldDelayTransientStatuses:	false
CorsParams:	*