Decide what to do with ECS support

[tobiasmcnulty]

ECS is problematic for a project like this, because typically you'll want more than just a web process running (celery or other background tasks, for example), but there's no way to define those with ECS without forking the stack and modifying the tasks in CloudFormation.

Elastic Beanstalk is a nice alternative because you can define the tasks outside of the infrastructure (in Dockerrun.aws.json).

We could leave ECS support in and just document the caveats, or strip it out altogether. I haven't yet found a reason to use it over any of the other app server options.

[tobiasmcnulty]

Closing in lieu of #107.