Overhaul with reasonable abstractions.

This seems like a reasonable division of code.  The "fio" package
continues to provide support for framed protocols through LimitedReader
and LimitedWriter while "zio" is a sample implementation for ZMTP/1.0.
Test and fix LimitedWriter.
Remove illogical caution.
Fixup zio and update docs.
Improve testing and documentation.

- Add doc.go for fio and zio.
- Add test for limited writer.
- Add README and example for zio.
- Use godocdown.