Added class documentation to the PLDocumentManager class.
Adding support for the document manager to keep track of when documents have changed. The document superclass now implements begin and end edit methods that lock the document. The endEdit method sends a PLDocumentWasEditedNotification.
The document architecture was changed such that more of the logic was handled by the document manager. Many document specific methods were moved to the abstract document superclass, and many others were removed from the documents altogether, transferring more responsibility to the document manager.

Initialization from a file on the system is implemented by the superclass, which then calls the setData: method. Subclasses of PLDocument must implement a setData: and documentData methods. These are used to read and write data from the filesystem. All subclasses must also implement isEqualToDocument:. The methods that PLDocument subclasses must implement are now specified in the PLDocument protocol.