adding 'unless' blocks

lib/grammar.js

@@ -594,6 +594,10 @@
     IfStart: [
       o("IF Expression Block", function() {
         return new IfNode($2, $3);
+      }), o("UNLESS Expression Block", function() {
+        return new IfNode($2, $3, null, {
+          invert: true
+        });
       }), o("IfStart ElsIf", function() {
         return $1.add_else($2);
       })

src/grammar.coffee

@@ -488,6 +488,7 @@ grammar: {
   # ambiguity.
   IfStart: [
     o "IF Expression Block",                    -> new IfNode $2, $3
+    o "UNLESS Expression Block",                -> new IfNode $2, $3, null, {invert: true}
     o "IfStart ElsIf",                          -> $1.add_else $2
   ]
 

test/test_if.coffee

@@ -33,3 +33,12 @@ else
   27
 
 ok result is 27
+
+
+# Testing unless.
+result: unless true
+  10
+else
+  11
+
+ok result is 11