Release highlights:

- Ensures compatibility with Rust 1.4 through 1.10.
- Support for Rust 1.0 through 1.3 has been dropped.
- Adds support for reading files with 32-bit IEEE float samples.

Apart from requiring a newer Rust version, this release includes a minor
breaking change: the `WavSpec` struct gained a new `sample_format`
member. To upgrade, add `sample_format: hound::SampleFormat::Int` to
places where a `WavSpec` is constructed.

Many thanks to Mitchell Nordine for his contributions to this release.