README: fixed headings level

Modified all h1 headings except the first one to h2, all h2 under h1 to h3,
etc.
Emit 'listening' and 'error' events

Modified DebugServer to emit 'listening' and 'error' events, added address()
function for getting the server's address. Moved console logs from DebugServer
to bin/inspector.js.

Modified CLI runner (bin/inspector.js) to forward DebugServer events to
the parent process via `process.send`.

Started embedding guide in `docs/embedding.md`.