Implemented generator.getDocumentPixmap

My attempt to solve export of document as one pixmap.

Minimal usage example (and test code) is here: https://github.com/marekhrabe/document-export. The example, immediately after initialization, gets `activeDocument.id` and calls `getDocumentPixmap` with that id and blank settings. It generates `document-pixmap.png` to current path.

Now it's on you to do the review :) 
Please add proper JSDoc to this method, including an "private" declaration, and documentation of the types of the formal parameters and return types.