Recursive json merge algorithm.

I've replaced the json concat function with a recursive merge algorithm. Thanks to Markus over at Stack Overflow (http://stackoverflow.com/a/383245/568916).

This was necessary since you can have resource files the current merge algorithm can not handle. E.g like this:
```
{
  "en": {
    "widget-a": {
      "title": "Activities",
      "has_been": "has been ",
      "completed": "completed",
      "added": "added",
      "deleted": "deleted",
      "reopened": "reopened"
    }
  }
}
```

Tests are included. Hope you like.