Explain proof-of-history in the readme

Also:
* Hash userdata so that verification works as the readme describes.
* Drop itertools package. Found a way to use std::iter instead.

Fixes #8

src/lib.rs

@@ -2,6 +2,5 @@
 pub mod log;
 pub mod historian;
 extern crate digest;
-extern crate itertools;
 extern crate rayon;
 extern crate sha2;