Implement subset of Node.js module API

Supports including modules from local files and HTTP. Also modules from
npm are supported (node_modules).

Limitations:

* No built-in modules yet (see [1]).
* No support for loading ".json" files as modules.
* No concept of a main module so far. The root require function is
  placed in the global scope.

For now, only the main module must be included in the "resources"
section of the index.json. Later, the main module could be defined
using the "main" entry in the project's index.json / package.json.

Still using eval instead of the native runInThisContext method as this
method is not yet fully supported on any client.

[1] http://nodejs.org/api/modules.html#modules_file_modules

Change-Id: I6608625be3ac13f635c3cd153977e977e2274ace