jhatwich/layer comps+size

To use this feature add a layer comp and name it something like "overview.png" or "200% overview.png"
It seems like this code ALWAYS initializes document objects that correspond to PSDs that have a layer comp. Why this is necessary? The intent is for the `DocumentManager` to only track changes for documents for which a client has called `getDocument`. Currently, this happens only for documents on which the Image Assets plugin has been enabled.