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
I don't think this needs to be an exported type because, as written, it's not possible to create one without using `NewBufferedWriter`.

We can have NewBufferedWriter return an `io.WriteCloseFlusher` interface (or whatever it's called)?