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/008-import.js

@@ -1,2 +1,2 @@
 
-require("pkg/js/parse_tests/import.js")
+require("./import.js")

pkg/js/parse_tests/020-complexRequire.js

@@ -0,0 +1 @@
+require('./complexImports/base.js');

pkg/js/parse_tests/020-complexRequire.json

@@ -0,0 +1,38 @@
+{
+  "registrars": [],
+  "dns_providers": [],
+  "domains": [
+    {
+      "name": "foo.com",
+      "registrar": "none",
+      "dnsProviders": {},
+      "records": [
+        {
+          "type": "A",
+          "name": "@",
+          "target": "1.2.3.4"
+        },
+        {
+          "type": "CNAME",
+          "name": "A",
+          "target": "foo.com."
+        },
+        {
+          "type": "CNAME",
+          "name": "C",
+          "target": "foo.com."
+        },
+        {
+          "type": "CNAME",
+          "name": "D",
+          "target": "foo.com."
+        },
+        {
+          "type": "CNAME",
+          "name": "B",
+          "target": "foo.com."
+        }
+      ]
+    }
+  ]
+}

pkg/js/parse_tests/complexImports/a/a.js

@@ -0,0 +1,3 @@
+function a() {
+    return CNAME("A", "foo.com.")
+}

pkg/js/parse_tests/complexImports/a/c/c.js

@@ -0,0 +1,8 @@
+require('../a.js');
+
+function c() {
+    return [
+        a(),
+        CNAME("C", "foo.com.")
+    ]
+}

pkg/js/parse_tests/complexImports/b/b.js

@@ -0,0 +1,8 @@
+require('pkg/js/parse_tests/complexImports/b/d/d.js');
+
+function b() {
+    return [
+        d(),
+        CNAME("B", "foo.com.")
+    ];
+}

pkg/js/parse_tests/complexImports/b/d/d.js

@@ -0,0 +1,3 @@
+function d() {
+    return CNAME("D", "foo.com.")
+}

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()
+);