Move Event into its own crate

The log crate was starting to be the catch-all for all things
related to entries, events, signatures, and hashes. This split
shows us that:

* Event depends only on signatures, not on hashes [directly]
* All event testing was done via log testing (shame on me)
* Accounting depends only on events

src/lib.rs

@@ -1,5 +1,6 @@
 #![cfg_attr(feature = "unstable", feature(test))]
 pub mod log;
+pub mod event;
 pub mod historian;
 pub mod accountant;
 pub mod accountant_skel;