add append/remove to modify graph node relations

[nikgraf]

I'm not sure but it, could look like

Append

var bart = graph.get('/person/bart'); // Data.Object
bart.append('locations', '/location/springfield');

Parameters of append:

• relation property name
• object _id
• (option with default false) boolean for adding back-link

Remove

bart.remove('locations', '/location/springfield');

Parameters of append:

• relation property name
• object _id
• (option with default false) boolean for removing back-link