Bug 990047 (1/2) - Add a shared function to load JSON files

Add a shared function to load JSON files
and use it throughout gaia to replace the
existing ad-hoc functions
Please add a comment describing what the function does. This is an example of how we document functions:

https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/js/about_more_info.js#L69

Don't forget to mention that if the file is not found the promise will be resolved with a ``null`` value.