This release focuses on improving write performance. Highlights:

- The header is now written when a `WavWriter` is constructed, therefore
  the constructor now returns a `Result`. This is a breaking change.
- `WavWriter` no longer maintains a buffer internally.
  `WavWriter::create()` does still wrap the file it opens in a buffered
  writer.
- Adds `SampleWriter16` for fast writing of 16-bit samples. Dedicated
  writers for other bit depths might be added in future releases.

Upgrading requires dealing with the `Result` in `WavWriter::new()`
and `WavWriter::create()`. In many cases this should be as simple as
wrapping the call in a `try!()`, or appending a `?` on recent versions
of Rust.