More complete XMLHttpRequest interface in Ajax

Includes content (request and response) abstraction and basic event handling.  I've moved the support functions into Unsafe like the other modules (though not all of them are necessarily unsafe).  There are still a few things missing, but there's enough to make it usable.  I'd add tests, but node doesn't support `XMLHttpRequest`.  It'd be nice to have better interfaces for things like `ArrayBuffer`, too, but this doesn't seem like the right place.