Changed styling (background color, centering stuff, positioning, etc.), and fixed some minor details like image URLs

* Background color for editor.js is now black
* You can now press 1-3 to select different tools quickly
* Toolbox doesn't appear in the middle of the map anymore
* Player list isn't associated with the toolbox--instead, it's positioned to the left of the map, where it was before.  This is because maps with many people would get fairly clunky toolboxes.
* Image URLs aren't massive data URLs anymore, and are hosted by the server properly.
* Index.html is now styled, but only very slightly.  I'm planning on making a very pretty & dynamic index page, but that will require research into how express handles POST requests, or major styling clientside.

Nothing's broken, as far as I can tell.  It also (IMO) looks prettier, but remember--you can easily revert anything I did if you don't like it.