add common.diff() function for test debugging

package.json

@@ -61,6 +61,7 @@
   },
   "devDependencies": {
     "ciao": "^0.6.0",
+    "difflet": "^1.0.1",
     "istanbul": "^0.4.2",
     "jshint": "^2.5.6",
     "nsp": "^2.2.0",

test/unit/run.js

@@ -1,6 +1,12 @@
+var tape = require('tape'),
+    diff = require('difflet')({ indent : 2, comment : true });
 
-var tape = require('tape');
-var common = {};
+var common = {
+  // a visual deep diff rendered using console.error()
+  diff: function( actual, expected ){
+    console.error( diff.compare( actual, expected ) );
+  }
+};
 
 var tests = [
   require('./controller/index'),