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/bin/client-demo.rs

@@ -4,7 +4,7 @@ fn main() {
     use silk::accountant_stub::AccountantStub;
     use std::time::Instant;
     use std::net::UdpSocket;
-    use silk::log::{generate_keypair, get_pubkey};
+    use silk::event::{generate_keypair, get_pubkey};
 
     let addr = "127.0.0.1:8000";
     let send_addr = "127.0.0.1:8001";