Refactor factory hierarchy to extend PolicyStatement

[alexcasalboni]

I thought of an alternative convention to get rid of the .build() and to make the code more readable.

Something like this:

    exampleFunction.addToRolePolicy(
      PolicyStatementFactory.buildPolicyStatement() // this returns a sub-class of the official PolicyStatement
        .setEffect(Effect.ALLOW)
        .addResource(exampleBucket.bucketArn)
        .addActions([Action.S3.LIST_BUCKET, Action.S3.PUT_OBJECT])
        // no final .build() here
    );

Basically, you wouldn't instantiate an object of class PolicyStatementFactory, but you'd use a static method to get an empty object of class GeneratedPolicyStatement, which would be a sub-class of PolicyStatement. I think this is also what @lusentis had in mind :)