Creation/modification time

Save _ctime (creation time) and _mtime (modification time) on each creation/modification of a document. Having creation/modification time for each document is convenient for a number of cases in desktop applications.

If adding two additional fields to a document is inconvenient, they can be automatically stripped off when returning a document and retrieved via a prototype function - .mtime(), .ctime() - so that they don't show up when enumerating the object's properties.