Caching (fixes #31), rewrite yolo in yolisp (fixes #27)

![](http://mylittlefacewhen.com/media/f/img/mlfw3330-1330548352244.gif)
basically:

    $env = [];
    $eval = function ($swag) use ($env) { return yolisp($swag, $env); };
    $env['foo'] = 'bar';
    $eval('foo'); // errors, it has old env