Add default timeout on deployment, in case stack gets stuck in UPDATE_IN_PROGRESS permanently

[kmanning]

We've seen issues in AWS, where a stack can get caught in UPDATE_IN_PROGRESS permanently. During a deployment, we poll the status for the stack, and wait until it either has a success, or a failure status - if the stack is perpetually in neither of those states, the polling (and therefore the deployment) will continue forever.

Add a default/configurable timeout around checking the stack status, so that such problems can be detected in an automated pipeline.

https://github.com/manheim/cf_deployer/blob/master/lib/cf_deployer/stack.rb#L156

Noted by @ryanrwalker during code review for Issue #38.