Issue #159 : caching system

* New Cache class with these methods:
 - setKey()
 - getKey()
 - isActivated()
 - flush()
 - isValid()
 - deleteKey()
 - getKeyPath()
 - getCachePath()
 - isObsolete()

* New constants:
 - CACHE constant activates/deactivates the cache
 - CACHE_TIME is the default duration of the cache
 - CACHE_PATH is the path to the cache folder

* GET['nocache'] to force a page not to use caching

* Cache duration
 - can't be older than generated data (uses a timestamp file in glossaire.sh)
 - can be defined in seconds when using getKey(), ex getKey($id, 300)

* Onestring model makes a use of the class, ex:

if (! $data = Cache::getKey($cache_id)) {
    /* create $data */
    /* Now cache data */
    Cache::setKey($cache_id, $data);
}