create simple pad server in node

this is a fully functional pad server with snapshots on the server.

it is not replicated or persistent - it's a single server, keeping everything in memory.

i think it's cool because

* JS is single threaded, so super easy to reason about server behavior
* it's only ~100 lines of code.

I'd like to do some scalability testing on this next, as well as improve the `getDiff` algorithm in `git` because it's really bumming me out how slow it is.