get cache created at NSDate

[prabirshrestha]

add a method that allows to get the date when the cache was created.

- (NSDate*) getCreatedAt:(NSString*)key;

It will then allow to use this logic.

if has cachedData
    load from cachedData
    if cachedCreateDate is old
        fetch online, cache it and reload
else
    fetch online, cache it and load

I want to set the cache as long as possible, but if it is old I would like to load from cache and fetch online too.