add remaining unhandled block types from the spec

padding blocks and reserved skippable blocks
Merge pull request #7 from bmatsuo/block-types

add remaining unhandled block types from the spec
simple BufferedWriter type

- 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.