Properly define node.js version, and reorganize package.json a bit

package.json

@@ -3,30 +3,34 @@
   "version":     "0.0.1",
   "keywords":    "base58",
   "description": "Base58 encoding and decoding",
-  "author": {
-    "name": "Jim Myhrberg",
-    "email": "contact@jimeh.me"
-  },
   "licenses": [{
     "type": "MIT",
     "url":  "https://raw.github.com/jimeh/node-base58/master/LICENSE.md"
   }],
 
+  "author": {
+    "name": "Jim Myhrberg",
+    "email": "contact@jimeh.me"
+  },
+
   "repository" : {
     "type" : "git",
     "url" : "http://github.com/jimeh/node-base58.git"
   },
 
   "main": "./lib/base58",
-  "engine": "node > 0.4.0 < 0.7.0",
 
-  "scripts": {
-    "test": "make test"
+  "engines": {
+    "node": ">= 0.4.0 < 0.7.0"
   },
 
   "devDependencies": {
     "coffee-script": "1.3.1",
     "mocha": "1.0.x",
     "should": "0.6.x"
+  },
+
+  "scripts": {
+    "test": "make test"
   }
 }