require() should handle paths as relative (like nodeJS) (#443)

* Integrate @nlowe's documentation and tests
* Use filepath.Clean() instead of filepath.Abs()
* Use a stackless implementation

pkg/js/parse_tests/complexImports/base.js

@@ -0,0 +1,8 @@
+require('./a/c/c.js');
+require('./b/b.js');
+
+D("foo.com","none",
+    A("@","1.2.3.4"),
+    c(),
+    b()
+);