Added support for asynchronous level

Hi,

I found Jasper was lacking of an asynchonous way to define levels.

Here is a fix for that. I tested everything i could with the current test config.

However, i suggest you to take a look to Sinon and sandboxing to be able to test console logs (asynchonously, this is the only way we have to give feedback to users). A blog post talking about that : https://nicolas.perriault.net/code/2013/testing-frontend-javascript-code-using-mocha-chai-and-sinon/

Hope you'll find it usefull.