D3 improve

[Neo-101-zz]

Inspired by eyaler, D3 visualization is improved. Following features are added:

• Node's color shows its citation group
• Node's size shows its times being cited (except root node)
• Click node to open its source website (except root node)
• Dragable nodes
• Zoom and pan
• Double-click to center node
• Resizable window
• Text labels
• Hover to highlight 1st-order neighborhood
• Click and press node to fade surroundings

Corresponding parts of README is modified too.

html output before modification:

html output after modification:

It seems that the title and author of nodes overlap. But user can easy distinguish them by panning, zooming and highlighting

[edsu]

This was merged, in some fashion in the end. Sorry it took so long.