Added support for arrays in nested attributes

This patch adds the possibility for arrays in nested attributes.

In the template you can now write:

    <input type="text" data-bind="groups.0.name">

Which will be interpreted as:

    { 
      "groups": [{
        "name": ""
      }]
    }

Previously it would be interpreted as an object:

    {
      "groups": {
        0: {
          "name": ""
        }
      }
    }