Throw helpful error message if class is initialized with non functions as methods.

Example of previous behavior (nodejs):
    var Joose = require('Joose');
    Joose.Class("C", {
      methods : {
        foo : null
      }
    });
    new C().foo();
------

    node.js:63
        throw e;
        ^
    TypeError: Cannot read property 'apply' of null
        at [object Object].foo (/Users/adam/.local/lib/node/.npm/joose/3.13.0/package/lib/Task/Joose/Core.js:752:13)
        at Object.<anonymous> (/Users/adam/mess/2010/42/triviabot/test.js:7:9)
        at Module._compile (node.js:462:23)
        at Module._loadScriptSync (node.js:469:10)
        at Module.loadSync (node.js:338:12)
        at Object.runMain (node.js:522:24)
        at Array.<anonymous> (node.js:755:12)
        at EventEmitter._tickCallback (node.js:55:22)
        at node.js:769:9