[isf-wifidog] caching and modification dates

[Benoit Gregoire]

Hi François,

I loved your patch for caching file objects (to fix 
http://dev.wifidog.org/ticket/135).  

I would like to generalize the modification and creation date functions and 
move them to the Content object as they are generic concepts.  In fact, there 
is already a creation date for every content object, but sadly there wasn't 
any accessor, so you probably didn't know.  

Any objections?