Cannot have multiple S3 event sources on same bucket for same Lambda Function.

[olabhrad]

The following configuration will not work:

event_sources:
- arn: arn:aws:s3:::my-bucket
events:
- s3:ObjectCreated:*
key_filters:
- type: prefix
value: my_path/
- arn: arn:aws:s3:::my-bucket
events:
- s3:ObjectCreated:*
key_filters:
- type: prefix
value: my_path2/

The problem is that Kappa is using the same notification id for both event sources.
def _make_notification_id(self, function_name): return 'Kappa-%s-notification' % function_name

This can be fixed by adding an optional "id" field for the s3 event source that can be utilised if there are more than one event sources on the same bucket e.g.

event_sources:
- arn: arn:aws:s3:::my-bucket
id: 1
events:
- s3:ObjectCreated:*
key_filters:
- type: prefix
value: my_path/
- arn: arn:aws:s3:::my-bucket
id: 2
events:
- s3:ObjectCreated:*
key_filters:
- type: prefix
value: my_path2/

[olabhrad]

PR: #120