Simplifying the use of asyncRequire.

When not using relative paths, it is now possible to do:

  var asyncRequire = require("noder-js/asyncRequire");
  asyncRequire("myModule").then(function(myModule){...})

The previous way still works:

  var asyncRequire = require("noder-js/asyncRequire").create(module);
  asyncRequire("myModule").then(function(myModule){...})

When using relative paths, it is now possible to do:

  var asyncRequire = require("noder-js/asyncRequire");
  asyncRequire(require.resolve("./myModule")).then(function(myModule){...});

The previous way still works:

  var asyncRequire = require("noder-js/asyncRequire").create(module);
  asyncRequire("./myModule").then(function(myModule){...});

However, the following use case should be avoided:

  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.

close #9