node-link repr. contains graph dict 'as is'

**Previously**
The `graph` attribute dictionary was previously split into a list of key-value pairs. This resulted in a counter-intuitive JSON representation with a structure different from the original `graph`-dict. The only upside was the ability to use numerical keys in the dictionary which is are supported by JSON but silently converted by `json.dumps(...)` from the `json` module.

**Now**
With the new approach the JSON representation matches the python dict and an note was included in the docstring which explains the new behavior.

This pull request is a follow up on this [discussion](https://groups.google.com/forum/#!searchin/networkx-discuss/json/networkx-discuss/xAJsOedGg1Q/-2uCZ4mE_EEJ) on the mailing list.