Calculate same-layer crossings

[anyazz]

Followup to pull request #271. Since edges between nodes with the same (manually determined) rank become possible, this adds a function for counting crossings within a single layer.
Since singleLayerCrossCount() requires graph to be defined using nodes, the second commit fixes failing tests in cross-count-test.js and adds a new one.

[Morragan]

Is this a complete solution for dagre to be able to compute a layout with nodes having the same rank? In that case it doesn't seem to work, I'm getting a following error, caused by y coordinate having a value of NaN:

Uncaught Error: Not possible to find intersection inside of the rectangle
    at Object.intersectRect (dagre.js:2865)
    at dagre.js:736
    at arrayEach (dagre.js:4497)
    at Object.forEach (dagre.js:9563)
    at assignNodeIntersects (dagre.js:723)
    at dagre.js:520
    at notime (dagre.js:2996)
    at runLayout (dagre.js:520)
    at dagre.js:477
    at notime (dagre.js:2996)