title: Can not find async after installation
body: <p>Today, I get strange thing that after i install async in global, nodejs reports it can not find the module.following is the workflow</p>
  
  <ol>
  <li><p>install async</p>
  
  <p>npm install -g async</p></li>
  <li><p>make sure async exists</p>
  
  <p>npm list -g async</p></li>
  </ol>
  
  <p>get this output:</p>
  
  <pre>
  /usr/local/lib
  ├── async@0.2.9
  └─┬ npm@1.3.21
    └─┬ request@2.30.0
      └─┬ form-data@0.1.2
        └── async@0.2.9 
  </pre>
  
  <p>3.try to use it.</p>
  
  <pre>
  I create a simple js file which only contains one statement:
  var async=require('async');
  then execute the file via node, I get exception:
  </pre>
  
  <pre>
  Error: Cannot find module 'async'
      at Function.Module._resolveFilename (module.js:338:15)
      at Function.Module._load (module.js:280:25)
      at Module.require (module.js:364:17)
      at require (module.js:380:17)
      at Object. (/lxzhu/nodejs/asynctest/test.js:1:73)
      at Module._compile (module.js:456:26)
      at Object.Module._extensions..js (module.js:474:10)
      at Module.load (module.js:356:32)
      at Function.Module._load (module.js:312:12)
      at Function.Module.runMain (module.js:497:10)
  </pre>