Fix type register visibility. Add a few more examples

examples/hello.rs

@@ -0,0 +1,10 @@
+extern crate rhai;
+use rhai::Engine;
+
+fn main() {
+    let mut engine = Engine::new();
+
+    if let Ok(result) = engine.eval("40 + 2".to_string()).unwrap().downcast::<i32>() {
+        println!("Answer: {}", *result);  // prints 42
+    }
+}