Add basic support for String.prototype.charAt() (#306)

* String - charAt() implementation

* String - charAt() - add js_class

tests/all/js_globals/JsString.rs

@@ -2,6 +2,35 @@
 
 use project;
 
+#[test]
+fn char_at() {
+    project()
+        .file("src/lib.rs", r#"
+            #![feature(proc_macro, wasm_custom_section)]
+
+            extern crate wasm_bindgen;
+            use wasm_bindgen::prelude::*;
+            use wasm_bindgen::js;
+
+            #[wasm_bindgen]
+            pub fn string_char_at(this: &js::JsString, index: u32) -> js::JsString {
+                this.char_at(index)
+            }
+        "#)
+        .file("test.ts", r#"
+            import * as assert from "assert";
+            import * as wasm from "./out";
+
+            var anyString = 'Brave new world';
+
+            export function test() {
+                assert.equal(wasm.string_char_at(anyString, 0), "B");
+                assert.equal(wasm.string_char_at(anyString, 999), "");
+            }
+        "#)
+        .test()
+}
+
 #[test]
 fn slice() {
     project()