Implement String#replace

Add an implementation for String.prototype.replace to [`String.c`](https://github.com/ndreynolds/flathead/blob/master/src/runtime/lib/String.c) and tests to [`test_string_global.js`](https://github.com/ndreynolds/flathead/blob/master/test/test_string_global.js) following the spec [here](http://es5.github.com/#x15.5.4.11).
