use the model name instead of file name as the hash key

This seems to be closer to how sequelize do things in their own documentation.

e.g.

```javascript
fs.readdirSync(__dirname)
  .filter(function(file) {                                    
    return (file.indexOf('.') !== 0) && (file !== 'index.js') 
  })                                                          
  .forEach(function(file) {                                   
    var model = sequelize.import(path.join(__dirname, file))  
    db[model.name] = model
  });

```