Populate: Pass alias attribute

I think, that populate should be passing aliases to joins. Aliases should be equal to model attribute name not the related model name.

Example:

Model User
```javascript
{
"attribute_name":{"model":"Group"}
}

User.populate('attribute_name')
```

Returns
```javascript
{"group":{data..}}
```

Should return
```javascript
{"attribute_name":{data..}}
```
Hope you get me.. You'll find sails-mysql part over [here](https://github.com/just-paja/sails-mysql/compare/balderdashy:associations...associations).

I woul like this to be more like .. asking for opinion than a pull request. So, what do you think?