Remove buffer.CountError.

It was only used by delete when given a negative count.
But delete is now an unexported detail of Write.
Write verifies that its addresses aren't bogus,
so delete should never see a bad count. Panic if it does.
Add two simple tests to cover two previsouly untested branches.
Mention the addition of literal newline matching.

This is a sam-ism that isn't documented in regexp(7).
Merge pull request #23 from eaburns/cleanups

Some small cleanups