Some small changes to help loading melonJS sources directly

I updated my local setup to load the latest melonJS source files automatically from the repository without having to build it. To make this a bit easier for me, I made two small changes which hopefully don't have any negative side effects.
First I extracted the list of source files from Gruntfile.js to avoid having to parse it.
Then I added a flag to core.js which prevents ```_init_ME();``` from being called before all source files are loaded, as I load them asynchronously.