Added initial modules support.

Added initial modules support.

The following syntax is supported:
1) default import statements:
	import lib1 from '../relative/path/lib.js';
	import lib2 from '/abs/path/lib.js';
	import fs from 'fs'; // built-in modules

2) default export statements:
	export default {fun1, fun2, ...}; // export module object

Modules look up procedure:
	1) absolute paths (start with '/') are used as is.
	2) otherwise the following paths are tried:
		dir + '/' + path where dir is
	  a) the directory of the current file.
	  b) additions paths provided with njs_vm_add_path().

This closes #91 on Github.