BlankLineStrippingWriter: clean up the code.

Writing to buffer always returns without error.
Remove unreachable return:
	go vet .
	util.go:80: unreachable code
BlankLineStrippingWriter: remove unused variable
BlankLineStrippingWriter: add test.
BlankLineStrippingWriter: return if io.Writer has an error.
BlankLineStrippingWriter: return something more meaningful.

Keep it simple and return the length of input, see
	http://golang.org/pkg/bytes/#Buffer.Write
If we can't write, it is a fatal error anyway.
BlankLineStrippingWriter: add read length test.
BlankLineStrippingWriter: fix comments.
Merge pull request #25 from capnm/upstream

Fix BlankLineStrippingWriter. Add tests.