Refactor context creation to a builder pattern

This mainly refactors context creation to use a builder pattern.  It
also adds a logdir option to this builder patter but doesn't yet use
this.

examples/simple.rs

@@ -39,8 +39,9 @@ fn main() {
     let dir = tempdir().unwrap();
     let dbfile = dir.path().join("db.sqlite");
     println!("creating database {:?}", dbfile);
-    let ctx =
-        Context::new(Box::new(cb), "FakeOs".into(), dbfile).expect("Failed to create context");
+    let ctx = ContextBuilder::new(Box::new(cb), "FakeOs".into(), dbfile)
+        .create()
+        .expect("Failed to create context");
     let running = Arc::new(RwLock::new(true));
     let info = ctx.get_info();
     let duration = time::Duration::from_millis(4000);