Updated Force Layout nodes and links

Different layouts have different definitions for node objects and link objects.
I created a new `GraphNodeForce` and `GraphLinkForce` interface for the Force Layout.
See the [docs](https://github.com/mbostock/d3/wiki/Force-Layout#nodes) for more info.
Merge pull request #2664 from styfle/patch-6

Updated Force Layout nodes and links