Created ability to select specific columns

Modified .all method to accept array of attributes

depending on the number of attributes given, the return type will be different. for example:

model.all({where: {name: 'bill'},attributes: ['id']},cb)
 will return an array of ids(e.g. ['1','2', ..])

model.all({where: {address: '123'}, attributes: ['id', 'name']},cb) 
will return [{id: 1, name: 'bill'}, {id: 2. name: 'alice'},...]