Add lambdaInvoke permissions for SNS Events

[tvandewalle]

SNS events need to be given the lambdaInvoke permission so that they can
call the lambda function. When the event is disabled/removed the permission
is also removed.

Fixes #76.

[olabhrad]

There is a an issue here. The add and remove functions are using the following:

StatementId=function.name+'_'+self._context.environment

This means that you can only add one SNS event source. On the second attempt you will get an error as you are using the same statement ID:

kappa.event_source.sns - DEBUG - Permission already exists - Continuing

Instead of using the function name, you could use the event source name which should be unique e.g.

StatementId=self.arn.split(":")[-1]

[olabhrad]

PR to fix above issue: #92