Add an example of wasm2asm and wasm-bindgen

This commit adds an example of executing the `wasm2asm` tool to generate asm.js
output instead of WebAssembly. This is often useful when supporting older
browsers, such as IE 11, that doesn't have native support for WebAssembly.

examples/asm.js/src/lib.rs

@@ -0,0 +1,16 @@
+#![feature(proc_macro, wasm_custom_section, wasm_import_module)]
+
+extern crate wasm_bindgen;
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+extern {
+    #[wasm_bindgen(js_namespace = console)]
+    fn log(s: &str);
+}
+
+#[wasm_bindgen]
+pub fn run() {
+    log("Hello, World!");
+}