Fix IE7 and IE8 compatibility

Commit [971a881](https://github.com/meltingice/ajax-chosen/commit/971a881f51d64b0db7fff82bb9dc561e1f671ce8) introduced a regression on Internet Explorer 7 & 8.

The Object.keys is not implemented on these browsers as described here: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Object/keys

I replaced this ECMAScript 5th Edition function by a more classic style. It's been tested on Internet Explorer.

Both Coffeescript & Javascript files have been modified, no need to compile.