Improve D3 visualization and README

[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

[p16i]

What do you think if we show the title and author names of certain nodes? For example, we can show only the attributes only for articles with more than x citations. For the other nodes, we show only when a mouse hovers on them.

[edsu]

omg, how did I miss this! I'm going to try to resolve the conflict and see if there is still a chance of merging this because it looks awesome!

[edsu]

I did merge this in and modified it pretty heavily, mostly to format the code, remove things that weren't being used, and then made some improvements like showing the citation direction, and using networkx for neighborhood detection. Thanks for the prod to making it better!