Render US map with connections!

Lots of work is included in this commit. Notably:

*   Use d3 to render a map of US
    *   Looked up geo coordinates of each powergrid US city and included
    *   Render cities & connections (minimally)
    *   Experimentally click & draggable
*   Use npm to manage dependency of d3.
    *   Currently using version 2.5.0, but I'd like to upgrade
*   Fix shameful typo of Cincinati