Added ability to query specific colums

Modified .all method to accept attributes property to select single or multiple columns.

<b>Example 1:</b>

model.all({where : {name: 'bill'}, attributes: 'id'}}, callback)
OR
model.all({attributes: 'id'}}, callback)

<i>returns array of ids (e.g. [ 1, 2 ]):</i>

<b>Example 2 : </b>

model.all({where : {name: 'bill'}, attributes: ['id']}}, callback) 
OR
model.all({attributes: ['id']}}, callback)

<i>returns array of objects(e.g. [{id: 1}, {id: 2}]):</i>

<b>Example 3: </b>

model.all({where: {address: '123'}, attributes: ['id', 'name']},callback) 
 OR
 model.all({, attributes: ['id', 'name']},callback) 

<i>returns [{id: 1, name: 'bill'}, {id: 2. name: 'alice'}]:</i>