Smart File Cache: Store and retrieve variable values in cache files

This class can be used to store and retrieve variable values in cache files.

It can serialize the value of a given variable and store it in a cache file.

The class can also retrieve the original variable value from a cache file. Cache files are valid for a given expiry period.

A special cache file can be used to store application wide variables. The variable values in this cache file are loaded every time an object of this class is created.

The access to cached variables can be done by accessing class variables with the same names as the cache keys.

The class may also explicitly remove a given cached variable file.