improve d3.js diagram
[d3 diagram] Add arrowheads to edges.
[d3 diagram] Fix nodes distances and diagram height.
[d3 diagram] Change font size.
[d3 diagram] Display facts and figures on nodes.
[d3 diagram] Improve facts and figures display.
[d3 diagram] Display neutral relationships correctly.
[d3 diagram] Limit fact and figures displayed on nodes to 4.
[d3 diagram] Redraw tspans on dragmove.
Merge pull request #2 from eea/d3_js_improvement

D3 js improvement