Add nested object capability to pointers

You currently can create a new object with

``` cpp
j["this"]["that"] = 27;
```

and if `this` and/or `that` don't exist, it's all good, it creates `this` as an object and puts `that` in it.

But if you use pointers (e.g. `"/this/that"`) it throws an exception if `this` doesn't exist.

For example:

``` cpp
j.at("/this/that"_json_pointer) = 27;
```

would be nice.
