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`.
fixup! improved error messages