Compatibility with Kint 1.x

When users are running jpgraph in a project which also uses Kint v1.x, polyfilling this class would break things, as reported in #63.

Now, in config.inc.php we check if Kint has property enabled_mode (in which case it's version 2.x) or method enabled in which case it's Kint v1.x. In either case, the debug mode is set accordingly.