WIP: Interledger Architecture

Based on the requests for information about the layers in Interledger and
how the different protocols relate to each other I've put together a
strawman over the weekend:

https://stackedit.io/viewer#!provider=gist&gistId=f534bfb53dfffb980c66&filename=interledger-architecture.md

There isn't much totally new stuff in there. It takes the ideas that are
floating around in our team, gives names to everything and makes a bunch of
arbitrary choices, all just to give us a starting point. Note that we're
still very actively working on this doc.

If you're easily confused by changing specs, maybe you'll want to wait
until the doc settles down a bit more before diving in. :)

Source for the doc is tracked on Github:
https://gist.github.com/justmoon/f534bfb53dfffb980c66