Allow multiple ARNs for a single event definition

[jnhmcknight]

Description

There was a bug with AWS event source handling when trying to setup triggers on the same function from multiple events. Only 1 of the events would be setup. In the original setup, the following zappa_settings.json would result in the behaviour described:

{
    "prod": {
        "s3_bucket": "my-zappa-packages",
        "role_name": "my-zappa-role",
        "cloudwatch_log_level": "ERROR",
        "events": [
            {
                "function": "my_app.handle_my_trigger",
                "event_source": {
                    "arn": "arn:aws:s3:::my-first-bucket",
                    "events": [
                        "s3:ObjectCreated:*"
                    ]
                }
            },
            {
                "function": "my_app.handle_my_trigger",
                "event_source": {
                    "arn": "arn:aws:s3:::my-second-bucket",
                    "events": [
                        "s3:ObjectCreated:*"
                    ]
                }
            },
            {
                "function": "my_app.handle_my_trigger",
                "event_source": {
                    "arn": "arn:aws:s3:::my-third-bucket",
                    "events": [
                        "s3:ObjectCreated:*"
                    ]
                }
            }
        ]
    }
}

Whereas, with these changes, the following settings work as expected, setting up the event source triggers on each of the buckets in question:

{
    "prod": {
        "s3_bucket": "my-zappa-packages",
        "role_name": "my-zappa-role",
        "cloudwatch_log_level": "ERROR",
        "events": [
            {
                "function": "my_app.handle_my_trigger",
                "event_source": {
                    "arn": [
                        "arn:aws:s3:::my-first-bucket",
                        "arn:aws:s3:::my-second-bucket",
                        "arn:aws:s3:::my-third-bucket"
                    ],
                    "events": [
                        "s3:ObjectCreated:*"
                    ]
                }
            }
        ]
    }
}

[coveralls]

Coverage decreased (-0.1%) to 74.225% when pulling 2715047 on jnhmcknight:multi-event-source into 7d38c15 on Miserlou:master.