Crate doesn't compile without `json` feature

[mbartlett21]

The crate's code uses extern crate serde_derive without #[cfg(feature = "json")], meaning that it doesn't compile if the json feature is disabled.

[llogiq]

I have started working on this (see #55), but there's a strange problem with the threads test. It works with the feature enabled, but not without it, and I haven't found the time to debug this.