jhatwich/layer comps+size

To use this feature add a layer comp and name it something like "overview.png" or "200% overview.png"
Log statements only useful for development should be removed. If this is actually an erroneous or catastrophic situation, it should either be logged (with the `Logger` object) as an error or an exception should be raised. 

Also: can this actually happen? I think we're traversing a set of comp (changes) here. I didn't think there was a notion of sections in the set of comps... Sorry if I'm misunderstanding.