fix wolfpack model require

When I create a file "test.js" in my root project folder (using Sails) with following content: 
<pre>
var wolfpack = require('wolfpack'),
      myModel = wolfpack('/api/models/MyModel');
</pre>
WolfPack cannot find MyModel file because it expects that file in wolfpack\lib\node_modules\api\models\MyModel
<pre>
try {
      model = require(model_path);
    }
</pre>

process.cwd() will get the current require function caller, make wolfpack search the right dir.