Changed last_modified calculation to be a bit more specific

Updated the README with details and provided examples.

Basically this refactor moves the calculation of last_modified to the Page/Post level, which allows for other Generator's Page/Post subclasses to provide a more accurate value.  It's a bit convoluted when it comes to integrating with other coupling to other Generators so I'll understand if you choose to ignore this, but I thought it was useful so here it is.