Diff algorithm for CloudFormation stack knowledge and current resource state

[afronski]

We would like to be able to see diff state between what CloudFormation knows about particular stack from its state (based on the file) and how the current resource looks like (after possible manual modification).

Such feature would ease restoring resources after manual modification in order to reapply safely CloudFormation updates.

[SylwiaGargula]

First version of this algorithm is on branch travis.