chore(bug): Update README + add async install

[kolyaventuri]

What

• Adds installAsync as a non-breaking alternative to `install``
  • Removes need for callback
• Updates README to reflect need for install before start

Why

Using the install method as-is results in a situation where at worst, the server cannot be started, and at best there's a race condition, as the following start method does not wait for completion of the installation, and would subsequently fail Code run in the above screenshot:

var dynamodbLocal = require('dynamodb-localhost');
dynamodbLocal.install();
dynamodbLocal.start({port: 8000});