Fix type register visibility. Add a few more examples

examples/simple_fn.rs

@@ -0,0 +1,16 @@
+extern crate rhai;
+use rhai::{Engine, FnRegister};
+
+fn add(x: i32, y: i32) -> i32 {
+    x + y
+}
+
+fn main() {
+    let mut engine = Engine::new();
+
+    &(add as fn(x: i32, y: i32)->i32).register(&mut engine, "add");
+ 
+    if let Ok(result) = engine.eval("add(40, 2)".to_string()).unwrap().downcast::<i32>() {
+       println!("Answer: {}", *result);  // prints 42
+    }
+}