[WIP] Use ES5 Object.defineProperty instead of __defineGetter__.

`__defineGetter__` is non standard and deprecated: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineGetter

`Object.defineProperty` is ES5 has good browser support: http://kangax.github.io/compat-table/es5/

WIP because there are two failing tests and I'm too lazy to debug right now:

    1) Gists forks an existing gist:
      Error: timeout of 10000ms exceeded
      at [object Object].<anonymous> (/home/ciro/bak/git/octokat.js/node_modules/mocha/lib/runnable.js:165:14)
      at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)

    2) Gists deletes a gist:
      Error: timeout of 10000ms exceeded
      at [object Object].<anonymous> (/home/ciro/bak/git/octokat.js/node_modules/mocha/lib/runnable.js:165:14)
      at Timer.listOnTimeout [as ontimeout] (timers.js:110:1