CloudWatch events - Fixes #52

[coopernurse]

Add support for CloudWatch events to the event_sources block. Also includes a cron sample that demonstrates usage.

I backed off of my approach of using type + name, in favor of the arn field, but I do think this approach is somewhat brittle as the user has to supply their account id in the arn and (I think) the arn needs to have rule/ in the prefix (e.g. rule/hello-cron)

But I think migration to a simplified event_sources format should be discussed separately, and for now I think it's ok to make this work like all the other sources.

One big thing to review -- in context.py I added this line to deploy():

self.add_event_sources()

This may be wrong, but I didn't see any path that would cause events to be registered -- create() was calling it, but when is that method called?

So I'm happy to remove that line before we merge this if that's incorrect - it will certainly impact code other than CloudWatch. But again, without that I'm not sure how any event sources will get added, but I may have missed how that works.

[josegonzalez]

You can drop the else clause here and just return None.

[coopernurse]

ok, that's done.

[josegonzalez]

Needs docs and tests!

[garnaat]

I originally left the add_event_sources out of deploy on purpose because most of the AWS examples show that as a separate step after doing some sort of mocked testing.

I'm ambivalent about it now. I think a good argument could be made for including it as part of deploy and I'm not opposed to that.