simple BufferedWriter type

```go
func (w *BufferedWriter) Close() error

func (w *BufferedWriter) Flush() error

func (w *BufferedWriter) Write(p []byte) (int, error)
```

- BufferedWriter writes blocks when flushed or MaxBlockSize bytes
  accumulate.

- Add tests and benchmarks corresponding to Writer's.

- Rewrite existing tests to demonstrate buffering misalignment.
  bytes.Reader/.Buffer types have optimizations which help in io.Copy.

Closes #3
You're *basically* enumerating all my reasons for *not* exporting the type at all, which I didn't think through, but I suppose that ship has sailed :grin:.

Let's just move forward as is, this is really such a simple API package anyway.