Update 03.answer.scala

Should be `&&` instead of `||`
Update and rename 19.answer.scala to 19.answer.markdown
Update 19.answer.markdown
Update 19.answer.markdown
Update 19.answer.markdown
Update README.md
Implement unfold via Option fns. Closes #325.
Merge pull request #330 from balajisivaraman/stream-unfold

Implement unfold via Option fns. Closes #325.
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/camford/fpinscala