Revert "Allow custom stream implementations to work with the reader"

This reverts commit 04504c3a049dabec193e443a9ff4cc752fe779b4.

Conflicts:
	reference-implementation/lib/readable-stream.js
Don't auto-release the reader on close for now.

Maybe we'll figure this out later.
Add a getReader() method anyway

It's probably a nicer user-facing API.