Simplifying the use of asyncRequire.

When not using relative paths, it is now possible to do:

```js
  var asyncRequire = require("noder-js/asyncRequire");
  asyncRequire("myModule").then(function(myModule){...})
```

The previous way still works:

```js
  var asyncRequire = require("noder-js/asyncRequire").create(module);
  asyncRequire("myModule").then(function(myModule){...})
```

When using relative paths, it is now possible to do:

```js
  var asyncRequire = require("noder-js/asyncRequire");
  asyncRequire(require.resolve("./myModule")).then(function(myModule){...});
```

The previous way still works:

```js
  var asyncRequire = require("noder-js/asyncRequire").create(module);
  asyncRequire("./myModule").then(function(myModule){...});
```
However, the following use case should be avoided:

```js
  var asyncRequire = require("noder-js/asyncRequire");
  asyncRequire("./myModule").then(function(myModule){...});
```

because then, the path is not relative to the current module, but to another (unspecified) one.