Accessing nested json attributes

We have added support for reading and editing nested json attributes. 
You can simply use dot separated format when defining columns.

Sample definition:
```javascript
var columns = {
  "nested.a": "A",
  "nested.b": editor({
     label: "B",
     editOn: "click"
  }),
  nestedc: {
     label: "C",
     field: "nested.c"
  }
}
```
This enhancement correctly reflects changes into data store as well.