Add CORS headers

[RyanHow]

Add CORS headers to allow AJAX calls from other origins to functions.

[RyanHow]

I'm working on this and will send a PR

[RyanHow]

Please see PR #638

[JhonatanHern]

It would be great to have an option in the dna.json that allows that, something like:

{
   "allowedOrigins" : [
      "localhost:3000"
   ]
}