Reorganize taskchampion crate for docs and tests

The public API of the taskchampion crate now contains the expected parts
and no more, and has some better documentation.

This moves the crate's external `tests/` into internal tests, as the
TaskDB is no longer exposed as part of the crate API.

cli/src/main.rs

@@ -1,6 +1,6 @@
 use clap::{App, Arg, SubCommand};
 use std::path::Path;
-use taskchampion::{taskstorage, Replica, Status, DB};
+use taskchampion::{taskstorage, Replica, Status};
 use uuid::Uuid;
 
 fn main() {
@@ -20,12 +20,9 @@ fn main() {
         .subcommand(SubCommand::with_name("gc").about("run garbage collection"))
         .get_matches();
 
-    let mut replica = Replica::new(
-        DB::new(Box::new(
-            taskstorage::KVStorage::new(Path::new("/tmp/tasks")).unwrap(),
-        ))
-        .into(),
-    );
+    let mut replica = Replica::new(Box::new(
+        taskstorage::KVStorage::new(Path::new("/tmp/tasks")).unwrap(),
+    ));
 
     match matches.subcommand() {
         ("add", Some(matches)) => {