Add support to METEOR_SETTINGS

[zvictor]

Based on the principles presented at http://joshowens.me/environment-settings-and-security-with-meteor-js/, we should be using METEOR_SETTINGS instead of sending our settings.json through Git.

I get an error, though:

> .scripts/test.js

[[[[[ ~/network/src ]]]]]

=> Started proxy.
=> Errors prevented startup:

   While preparing to run:
   settings.json: file not found (settings file)

[zvictor]

Right now I am fixing it with the following:

test:
  pre:
    - echo $METEOR_SETTINGS > src/settings.json

I haven't tried a safer option to avoid problems when the settings.json already exists, therefore I am not creating a PR for this yet.

[ghost]

I think using Object.assign instead of require('util')._extend will fix this.