Issue #159 : caching system

* New Cache class with these methods:
 Public:
 - setKey()
 - getKey()
 - flush()

 Private:
 - deleteKey()
 - isValidKey()
 - isObsoleteKey()
 - getKeyPath()
 - getCachePath()
 - isActivated()
* New constants:
 - CACHE_ENABLED activates/deactivates the cache
 - CACHE_TIME is the default duration of the cache
 - CACHE_PATH is the path to the cache folder
There are also fallback constants in the class to use it outside of Transvision as a drop-in class.
* GET['nocache'] to force a page not to use caching
* Cache duration
 - age is the time elapsed since last generation of data, 5h20 if it can't be determined (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);
}

* Tests for all methods